Bar code symbol scanning system having a holographic laser scanning disc utilizing maximum light collection surface area thereof and having scanning facets with optimized light collection efficiency

ABSTRACT

A bar code symbol scanning system having a holographic laser scanning disc utilizing maximum light collection surface area thereof and having scanning facets with optimized light collection efficiency. The holographic scanning disc has a plurality of holographic optical elements for scanning a laser beam and producing a laser scanning pattern for scanning code symbols. Each holographic optical element being supported on a support disc between the inner and outer perimeters thereof, and each has a surface area for carrying out light collecting operations and at least a portion of the surface area is disposed adjacent the outer perimeter of the support disc for carrying out laser beam scanning operations. The sum of all of the facet surface areas of the plurality of said holographic optical elements is substantially equal to the surface area of the available light collecting region of the support disc.

RELATED CASES

This is a Continuation of copending application Ser. No. 08/886,806filed Apr. 22, 1997, which is a Continuation-in-Part of application Ser.Nos. 08/573,949 filed Dec. 18, 1995 now abandoned; 08/615,054 filed Mar.12, 1996; 08/476,069 filed Jun. 7, 1995 now U.S. Pat. No. 5,591,353;08/561,479 filed Nov. 22, 1995 now U.S. Pat. No. 5,661,292; which is acontinuation of Ser. Nos. 08/293,695 filed Aug. 19, 1995 now U.S. Pat.No. 5,468,951; 08/293,493 filed Aug. 19, 1994 now U.S. Pat. No.5,525,789; 08/475,376 filed Jun. 7, 1995 now U.S. Pat. No. 5,637,852;08/439,224 filed May 11, 1995 now U.S. Pat. No. 5,627,359; and08/292,237 filed Aug. 17, 1994 now U.S. Pat. No. 5,808,285, eachcommonly owned by Assignee, Metrologic Instruments, Inc., of Blackwood,N.J., and is incorporated herein by reference as if fully set forthherein.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates generally to holographic laser scanners ofultra-compact design capable of reading bar and other types of graphicalindicia within a large scanning volume using holographic opticalelements and visible laser diodes, and also a method of designing andoperating the same for use in diverse applications.

2. Brief Description of the Prior Art

The use of bar code symbols for product and article identification iswell known in the art. Presently, various types of bar code symbolscanners have been developed. In general, these bar code symbol readerscan be classified into two distinct groups.

The first class of bar code symbol reader simultaneously illuminates allof the bars and spaces of a bar code symbol with light of a specificwavelength(s) in order to capture an image thereof forrecognition/decoding purposes. Such scanners are commonly known as CCDscanners because they use CCD image detectors to detect images of thebar code symbols being read.

The second class of bar code symbol reader uses a focused light beam,typically a focused laser beam, to sequentially scan the bars and spacesof a bar code symbol to be read. This type of bar code symbol scanner iscommonly called a “flying spot” scanner as the focused laser beamappears as “a spot of light that flies” across the bar code symbol beingread. In general, laser bar code symbol scanners are subclassifiedfurther by the type of mechanism used to focus and scan the laser beamacross bar code symbols.

The majority of laser scanners in use today employ lenses and moving(i.e. rotating or oscillating) mirrors in order to focus and scan laserbeams across bar code symbols during code symbol reading operations.Examples of such laser scanners are disclosed in great detail in theBackground of Invention of U.S. Pat. Nos. 5,216,232 to Knowles et al.;5,340,973 to Knowles et al.; 5,340,971 to Rockstein et al.; 5,424,525 toRockstein et al., which are incorporated herein by reference.

One type of laser scanner that has enjoyed great popularity in recentyears is called the “polygon scanner” in that it employs a rotatingpolygon whose sides bear light reflective surfaces (e.g. mirrors) forscanning a laser beam over multiple paths through space above thescanning window of the scanner. In polygon-type laser scanners, theangular sweep of the outgoing laser beam and the light collectionefficiency of the return laser beam are both directly related to thenumber and size of light reflective facets on the rotating polygon.

In contrast to laser scanners, which use lenses (i.e. light refractiveelements) to shape and focus laser light beams and light reflectivesurfaces to scan focused laser beams, there exists another subclass oflaser scanner which employs a high-speed holographic disc. In general,the holographic disc comprises an array of holographic optical elements(HOEs) called “facets” which function to focus and deflect outgoinglaser beams during laser beam scanning operations, as well as focusincoming reflected laser light during light collection/detectionoperations. Such bar code symbol scanners are typically calledholographic laser scanners or readers because holographic opticalelements (HOEs) are employed. Examples of prior art holographic scannersare disclosed in U.S. Pat. Nos. 4,415,224; 4,758,058; 4,748,316;4,591,242; 4,548,463; 5,331,445 and 5,416,505, incorporated herein byreference.

Holographic laser scanners, or readers, have many advantages over laserscanners which employ lenses and mirrors for laser beam focusing andscanning (i.e. deflection) functions.

One of the major advantages of holographic laser scanners over polygonlaser scanners is the ability of holographic laser scanners toindependently control (i) the angular sweep of the outgoing laser beamand (ii) the light collection efficiency for the returning laser beam.

Holographic laser scanners have other advantages over polygon-type laserscanners. In particular, in holographic laser scanners, light collectionefficiency is determined by the size of the light collecting portion ofeach holographic facet, while the angular sweep of the outgoing laserbeam is determined by the angular width of the outgoing beam portion ofthe holographic facet and the angles of incidence and diffraction of theoutgoing laser beam.

While prior art holographic scanning systems have many advantages overmirror-based laser scanning systems, prior art holographic scanners arenot without problems.

In the first holographic scanner produced by International BusinessMachines (IBM), the holographic facets on its holographic disc weresimple sectors which did not allow for independent control over lightcollection and light scanning functions. Consequently, such holographicscanners had faster scanning speeds than were needed for theapplications at hand. Subsequent industrial scanners designed by IBMallowed independent control of these functions. However, the holographicdiscs employed in prior art holographic scanners, e.g. the HOLOSCAN2100™ holographic laser scanner designed and sold by Holoscan, Inc. ofSan Jose, Calif., fail to (i) maximize the use of available space on thedisc for light collection purposes, and (ii) minimize the scan linespeed for particular laser scanning patterns. As a result of such designlimitations, prior art holographic scanners have required the use oflarge scanning discs which make inefficient use of the available lightcollecting surface area thereof. They also are incapable of producingfrom each holographic facet thereon, detected scan data signals havingsubstantially the same signal level independent of the location in thescanning volume from which the corresponding optical scan data signal isproduced. Consequently, this has placed great demands on the electricalsignal processing circuitry required to handle the dramatic signalswings associated with such detected return signals.

While U.S. Pat. No. 4,415,224 to Applicant (Dickson) discloses a methodof equalizing the light collection efficiency of each facet on theholographic scanning disc, it does not disclose, teach or suggest amethod of equalizing the light collection efficiency of each facet onthe holographic scanning disc, while utilizing substantially all of thelight collecting surface area thereof. Thus, in general, prior artholographic laser scanners have required very large scanner housings inorder to accommodate very large scanning discs using only a portion oftheir available light collection surface area.

In many code symbol reading applications, the volumetric extent of theholographic scanner housing must be sufficiently compact to accommodatethe small volume of space provided for physical installation. However,due to limitations of conventional design principles, it has not beenpossible to build prior art holographic scanners having sufficientcompactness required in many applications. Consequently, the hugehousings required to enclose the optical apparatus of prior artholographic laser scanners have restricted their use to only a fewpractical applications where housing size constraints are of littleconcern.

While highly desirable because of their low power usage and miniaturesize, solid-state visible laser diodes (VLDs) cannot be used practicallyin prior art holographic laser scanners because of several problemswhich arise from inherent properties of conventional VLDs.

The first problem associated with the use of VLDs in holographic laserscanners is that the VLDs do not produce a single spectral line outputin the manner of conventional He—Ne laser tubes. Rather, conventionalVLDs always produce some background super-luminescence, which is a broadspectrum of radiation of the type produced by conventional lightemitting diodes (LEDs). Also, VLDs often operate in more than oneoscillation mode and/or exhibit mode hopping, in which the VLD jumpsfrom one mode of oscillation to another. Both of these characteristicsof VLDs result in a spreading of the laser beam as it leaves the highlydispersive holographic facet of the holographic disc. This results in aneffectively larger “spot” at the focal point of the holographic facet,causing errors in the resolution of the bars and spaces of scanned codesymbols and, often, intolerable symbol decoding errors.

The second problem associated with the use of VLDs in a holographicscanner is that the inherent “astigmatic difference” in VLDs results inthe production of laser beams exhibiting astigmatism along thehorizontal and vertical directions of propagation. This fact results inthe outgoing laser beam having a cross-sectional dimension whose sizeand orientation varies as a function of distance away from the VLD.Thus, at particular points in the scanning field of a holographicscanner using a VLD, the orientation of the laser beam (“flying spot”)will be such that the bars and spaces cannot be resolved for symboldecoding operations.

Holographic scanners suffer from other technical problems as well.

In prior art holographic scanners, the light collection and detectionoptics are necessarily complicated and require a significant volume ofspace within the scanner housing. This necessarily causes the heightdimension of the scanner housing to be significantly larger than desiredin nearly all code symbol reading applications.

When an outgoing laser beam passes though, and is diffracted by, therotating holographic facets of prior art holographic scanners,“holographically-introduced” astigmatism is inherently imparted to theoutgoing laser beam. While the source of this type of astigmatism isdifferent than the source of astigmatism imparted to a laser beam due tothe inherent astigmatic difference in VLDs, the effect is substantiallythe same, namely: the outgoing laser beam has a cross-sectionaldimension whose size and orientation varies as a function of distanceaway from the holographic facet. Thus, at particular points in thescanning field of a holographic scanner, the orientation of the laserbeam (i.e. “the flying spot”) will be such that the bars and spaces of ascanned bar code symbol cannot be resolved for symbol decodingoperations. Consequently, it has been virtually impossible to design aholographic laser scanner with a three-dimensional scanning volume thatis capable of scanning bar code symbols independent of their orientationas they move through the scanning volume.

Because of the methods used to design and construct prior artholographic disks, the size and shape of the light collection area ofeach facet could not be controlled independent of the angular sweep ofthe outgoing laser beam. Consequently, this has prevented optimal use ofthe disk surface area for light collection functions, and thus theperformance of prior art holographic scanners has been necessarilycompromised.

While the above problems generally define the major areas in whichsignificant improvement is required of prior art holographic laserscanners, there are still other problems which have operated to degradethe performance of such laser scanning systems.

In particular, glare produced by specular reflection of a laser beamscanning a code symbol reduces the detectable contrast of the bars andspaces of the symbol against its background and thus the SNR of theoptical scan data signal detected at the photodetectors of the system.While polarization filtering techniques are generally known foraddressing such problems in laser scanning systems, it is not known howsuch techniques might be successfully applied to holographic type laserscanning systems while simultaneously solving the above-describedproblems.

Thus, there is a great need in the art for an improved holographic laserscanning system and a method of designing and constructing the same,while avoiding the shortcomings and drawbacks of prior art holographicscanners and methodologies.

OBJECTS AND SUMMARY OF THE PRESENT INVENTION

Accordingly, a primary object of the present invention is to provide aholographic laser scanner free of the shortcomings and drawbacks ofprior art holographic laser scanning systems and methodologies.

Another object of the present invention is to provide a holographiclaser scanner which produces a three-dimensional laser scanning volumethat is substantially greater than the volume of the housing of theholographic laser scanner itself, and provides full omni-directionalscanning within the laser scanning volume.

A further object of the present invention is to provide such aholographic laser scanner, in which the three-dimensional laser scanningvolume has multiple focal planes and a highly confined geometryextending about a projection axis extending from the scanning window ofthe holographic scanner.

A further object of the present invention is to provide such aholographic laser scanner, in which a plurality of symmetricallyarranged laser diodes are used to simultaneously produce a plurality oflaser beams which are focused and scanned through the scanning volume bya plurality of volume-transmission type holographic optical elements,each of which is supported upon a centrally located rotating disc andparticularly designed to produce a single scanning plane of a particulardepth of focus when one of the laser beams passes therethrough duringthe operation of the holographic laser scanner.

A further object of the present invention is to provide such aholographic laser scanner, in which laser light produced from aparticular holographic optical element reflects off a bar code symbol,passes through the same holographic optical element, and is thereaftercollimated for light intensity detection.

A further object of the present invention is to provide such aholographic laser scanner, in which a plurality of lasers simultaneouslyproduce a plurality of laser beams which are focused and scanned throughthe scanning volume by a rotating disc that supports a plurality ofholographic facets.

A further object of the present invention is to provide such aholographic laser scanner, in which the scanner housing has an aperturedscanning window which allows simultaneously projection of multiplescanning planes, at angles which differ from each other over theduration of each scanning pattern generation cycle.

A further object of the present invention is to provide such aholographic laser scanner, in which the holographic optical elements onthe rotating disc maximize the use of the disk space for lightcollection, while minimizing the laser beam velocity at the focal planesof each of the laser scan patterns, in order to minimize the electronicbandwidth required by the light detection and signal processingcircuitry.

A further object of the present invention is to provide a compactholographic laser scanner, in which substantially all of the availablelight collecting surface area on the scanning disc is utilized and thelight collection efficiency of each holographic facet on the holographicscanning disc is substantially equal, thereby allowing the holographiclaser scanner to use a holographic scanning disc having the smallestpossible disc diameter.

A further object of the present invention is to provide a compactholographic laser scanner, in which the beam steering portion of eachholographic facet on the holographic scanning disc is provided with alight diffraction efficiency that is optimized for an incident laserbeam having a first polarization state, whereas the light collectingportion of each holographic facet is provided with a light diffractionefficiency that is optimized for reflected laser light having a secondpolarization state orthogonal to the first polarization state, whilelight focused onto the photodetectors of the system are passed throughpolarization filters which transmit collected laser light having thesecond polarization state and block collected laser light having thefirst polarization state.

A further object of the present invention is to provide such aholographic laser scanner, in which laser beam astigmatism caused by theinherent astigmatic difference in each visible laser diode iseffectively eliminated prior to the passage of the laser beam throughthe holographic optical elements on the rotating scanning disc.

A further object of the present invention is to provide such aholographic laser scanner, in which the dispersion of the relativelybroad spectal output of each visible laser diode by the holographicoptical elements on the scanning disc is effectively automaticallycompensated for as the laser beam propagates from the visible laserdiode, through an integrated optics assembly, and through theholographic optical elements on the rotating disc of the holographiclaser scanner.

A further object of the present invention is to provide such aholographic laser scanner, in which a conventional visible laser diodeis used to produce a laser scanning beam, and a simple and inexpensivearrangement is provided for eliminating or minimizing the effects of thedispersion caused by the holographic disc of the laser scanner.

A further object of the present invention is to provide such aholographic laser scanner, in which the inherent astigmatic differencein each visible laser diode is effectively eliminated prior to the laserbeam passing through the holographic optical elements on the rotatingdisc.

A further object of the present invention is to provide such aholographic laser scanner, in which the laser beam produced from eachlaser diode is processed by a single, ultra-compact optics module inorder to circularize the laser beam produced by the laser diode,eliminate the inherent astigmatic difference therein, as well ascompensate for wavelength-dependent variations in the spectral output ofeach visible laser diode, such as superluminescence, multi-mode lasing,and laser mode hopping, thereby allowing the use of the resulting laserbeam in holographic scanning applications demanding large depths offield.

A further object of the present invention is to provide such aholographic laser scanner, in which the focal lengths of the multiplefocal regions of the laser scanning volume are strategically selected soas to create an overlap at the ends of the scanning planes in the nearand far regions of adjacent focal regions in the scanning volume, makingit easier to read a bar code symbol passing therethrough independent ofits orientation.

A further object of the present invention is to provide such aholographic laser scanner, in which an independent lightcollection/detection subsystem is provided for each laser diode employedwithin the holographic laser scanner.

A further object of the present invention is to provide such aholographic laser scanner, in which the geometrical dimensions of itsbeam folding mirrors in conjunction with the geometrical dimensions ofits holographic disc are the sole determinants of the width and lengthdimensions of the scanner housing, whereas the geometrical dimensions ofits beam folding mirrors and parabolic light collecting mirrors beneaththe holographic disc are the sole determinants of the height dimensionof the scanner housing.

A further object of the present invention is to provide such aholographic laser scanner, in which an independent signal processingchannel is provided for each laser diode and light collection/detectionsubsystem in order to improve the signal processing speed of the system.

A further object of the present invention is to provide such aholographic laser scanner, in which a plurality of signal processors areused for simultaneously processing the scan data signals produced fromeach of the photodetectors within the holographic laser scanner.

A further object of the present invention is to provide such aholographic laser scanner, in which each facet on the holographic dischas an indication code which is encoded by the zero-th diffraction orderof the outgoing laser beam and detected so as to determine whichscanning planes are to be selectively filtered during the symboldecoding operations.

A further object of the present invention is to provide such aholographic laser scanner, in which the zero-th diffractive order of thelaser beam which passes directly through the respective holographicoptical elements on the rotating disc is used to produce a start/homepulse for use with stitching-type decoding processes carried out withinthe scanner.

A further object of the present invention is to provide a code symbolreading system in which a holographic laser scanner is used to create ascanning volume within which the presence of a code symbol is detected,and a high speed laser scanner is used to scan the region within whichthe detected bar code resides, to collect high-resolution scan data fordecode processing.

A further object of the present invention is to provide ahand-supportable, hand-mounted and body-wearable scanning deviceemploying a holographic scanning mechanism to create various types ofscanning patterns, including 2-D raster patterns, within a 3-D scanningvolume.

A further object of the present invention is to provide a novel methodof designing such a holographic laser scanner having a housing with aminimum height (i.e. depth) dimension for any given three-dimensionallaser scanning pattern confined within a specified scanning volumeduring bar code symbol reading operations.

A further object of the present invention is to provide a novel methodof designing a holographic disk for such a holographic laser scanner,such that both the size and shape of the light collection area of eachholographic optical element (i.e. facet) on the rotating disc iscontrolled independent of the angular sweep of the outgoing laser beamin order to make maximum use of the disk surface area for lightcollection functions during the laser scanning process.

A further object of the present invention is to provide a novel methodof designing a laser beam optics module for use with the holographicscanning disc and laser diode employed in the holographic laser scannerhereof, which functions to circularize the laser beam produced from thelaser diode, eliminate the inherent astigmatic difference therein, andcompensate for wavelength-dependent variations in the spectral output ofthe visible laser diode, such as superluminescence, multi-mode lasing,and laser mode hopping.

A further object of the present invention is to provide a novel methodof designing a holographic disc for a holographic laser scanner, inwhich all of the available area on the disk is used for optimizing thelight collection efficiency thereof and thus improve the performance ofthe holographic laser scanner.

A further object of the present invention is to provide such disc designmethod, in which to determine the sizes and shapes of the holographicfacets thereof, a 3-D surface geometry program is used to create a 3-Dgeometrical model of the components of the holographic laser scanner andits 3-D laser scanning pattern, whereas a spreadsheet modelling programis used to create an analytical model for the holographic laser scannerand its 3-D laser scanning pattern.

A further object of the present invention is to provide such disc designmethod which employs a spreadsheet-type computer program for creatinganalytical model of the process of generating a prespecified laserscanning pattern using a prespecified holographic facet support disc andbeam folding mirror arrangement, and arriving at an optimal set ofholographic facet parameters which, for prespecified size holographicfacet support disc, minimizes the heightwise, lengthwise and widthwisedimensions of the scanner housing.

These and other objects of the present invention will become apparenthereinafter and in the claims to Invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to more fully understand the Objects of the Present Invention,the following Detailed Description of the Illustrative Embodimentsshould be read in conjunction with the accompanying Figure Drawings inwhich:

FIG. 1A is a perspective view of the holographic laser scanning systemof the present invention shown installed in a first exemplaryapplication environment;

FIG. 1B is a perspective view of the holographic laser scanning systemof the present invention shown installed in a second exemplaryapplication environment;

FIG. 1C is a perspective view of the holographic laser scanning systemof the present invention shown installed in a third exemplaryapplication environment;

FIG. 2A is a global perspective view of the holographic scanning systemof the illustrative embodiment of the present invention, shown with itshousing and the light detector support structure removed from itsoptical bench in order to reveal the holographic scanning disc, beamfolding mirrors, laser beam production modules, analog/digital signalprocessing boards, and other structures otherwise hidden by the housingand the light detector support structure of the system;

FIG. 2B is limited perspective view of the holographic scanning systemof the illustrative embodiment, showing in greater detail the beamfolding mirror of the first scanning channel of the system, in relationto its associated laser beam production module, parabolic lightcollection mirror, photodetector and analog/digital signal processingboard, arranged about the centrally rotating holographic scanning discof the system;

FIG. 2C is a partially cut away elevated side view of the holographicscanning system of the illustrative embodiment, showing in greaterdetail from about the holographic disc, the laser beam productionmodule, beam folding mirror, parabolic light detection mirror andphotodetector associated with one laser station of the system of thepresent invention;

FIG. 2D is a partially cut away view of the holographic scanning systemof the illustrative embodiment, taken along line 2D—2D of FIG. 2C,showing in greater detail the holographic scanning disc, the arrangementof the beam folding mirror and parabolic light detection mirrorassociated with an illustrative laser scanning station of the system ofthe present invention;

FIG. 2E is a perspective view of the holographic scanning system of theillustrative embodiment, showing the scanning window array of thescanner housing of the present invention;

FIG. 3 is a plan view of the holographic scanning disc of theillustrative embodiment of the present invention, showing the boundariesof each i-th holographic optical facet mounted thereon about its axis ofrotation, with the assigned facet number imposed thereon forillustrative purposes;

FIGS. 4A, 4B and 4C set forth a block functional diagram of holographiclaser scanning system of the illustrative embodiment of the presentinvention, showing the major components of the system and their relationto each other;

FIG. 5 is a perspective view of the holographic laser scanning system ofthe illustrative embodiment of the present invention, schematicallyillustrating the projection of each P(i,j)-th laser scanning plane atits prespecified focal plane (i.e. zone) within the three-dimensionalscanning volume extending about the projection axis of the holographiclaser scanner;

FIG. 5A is a schematic diagram showing the time order in which eachP(i,j)-th laser scanning plane is cyclically generated as the j-th laserbeam passes through the i-th holographic facet on the rotatingholographic scanning disc within the scanner housing during laserscanning operations;

FIG. 6A is a schematic diagram showing the overlapping nature of thescanlines produced from different holographic facets, betweenspatially-adjacent focal planes within the laser scanning volumeprojected from the holographic laser scanner of the present invention;

FIGS. 6B and 6C are schematic diagrams illustrating the various beamcross-sections of two laser scanning beams having focal lengths in thefar portion of the scanning volume, shown at a number of differentpoints along their respective scanline trajectories as well as betweentheir respective adjacent focal planes, showing astigmatic laser beamoverlapping within each interfocal plane region of the three-dimensionallaser scanning pattern;

FIG. 7 is a flow chart illustrating the major steps involved in themethod of designing the holographic disc and laser beam productionmodule(s) of the holographic scanning system of the present invention;

FIG. 8A is a geometrical optics model of the process of producing theP(i,j)-th laser scanning plane (i.e. P(i,j)-th laser scanline) locatedwithin the three-dimensional scanning volume of the holographic scanningsystem hereof, by directing the j-th laser beam through i-th holographicfacet supported on the rotating holographic scanning disc thereof;

FIG. 8A1 is a view of the geometrical optics model of FIG. 8A, showingparticular parameters in greater detail;

FIG. 8B1 and FIGS. 8B2 and 8B3, collectively, show a table listing theparameters used to represent the geometrical optics model of FIGS. 8Aand 8A1;

FIGS. 8C1 and 8C2, collectively, show a table listing the mathematicalequations describing structural and functional relationships amongparticular parameters of the geometrical optics model of FIGS. 8A and8A1;

FIG. 9 is a schematic diagram of the holographic scanning disc of theillustrative embodiment designed according to the method of the presentinvention, and indicating the various geometrical parameters used tospecify the geometrical characteristics of each i-th holographic facetthereof;

FIG. 10A1 is a geometrical optics model illustrating the path travelledby the light rays associated with an incident laser beam being initiallydiffracted by a rotating holographic facet towards a bar code symbol,then returning light rays reflected therefrom being diffracted again bythe same holographic facet towards a light focusing parabolic mirror,and finally the focused light rays being transmitted through the sameholographic facet towards its photodetector without diffraction;

FIGS. 10A2 and 10A3 set forth geometrical optics models of the processof a laser beam propagating through a holographic facet on the rotatingholographic scanning disc shown in FIG. 10A1, which are used during thedisc design process hereof to compute the normalized total out-and-backlight diffraction efficiency of each holographic facet to S and Ppolarized light when no cross-polarizer is used in the holographic laserscanner;

FIG. 10B sets forth a set of parameters used to represent thegeometrical optics models of FIGS. 10A1, 10A2, and 10A3;

FIG. 10B1 sets forth a set of initialized (i.e. assumed) values forvarious parameters used in the geometrical optics models of FIGS. 10A1,10A2, and 10A3;

FIG. 10C1 sets forth a set of mathematical expressions describingstructural and functional relationships among particular parameters ofthe geometrical optics model of FIGS. 10A1, 10A2, and 10A3;

FIG. 10C2 sets forth a set of equations defining (1) the lightdiffraction efficiency of the i-th holographic scanning facet toS-polarized outgoing light rays incident on the holographic scanningdisc, (2) the light diffraction efficiency of the i-th holographicscanning facet to P-polarized outgoing light rays incident on theholographic scanning disc, and (3) the total out-and-back lightdiffraction efficiency of the i-th holographic scanning facet toS-polarized outgoing light rays incident on the holographic disc, eachbeing expressed as a function of the modulation-depth (i.e.modulation-index) within a fixed thickness gelatin;

FIG. 10D sets forth a set of equations used to calculate both Fresnellosses and transmission of P and S polarized light rays passing throughthe holographic scanning facets, for use in the light diffractionefficiency expression set forth in FIG. 10C2;

FIG. 10E1 sets forth a set of graphs plotting, as a function of themodulation-depth (i.e. modulation-index) within a fixed thicknessgelatin, (1) the light diffraction efficiency of the first holographicscanning facet to S-polarized outgoing light rays incident thereto, (2)the light diffraction efficiency of the first holographic scanning facetto P-polarized outgoing light rays incident thereto, and (3) the totalout-and-back light diffraction efficiency of the first holographicscanning facet to S-polarized outgoing light rays incident, which areultimately used to compute the total out-and-back light diffractionefficiency of the first holographic facet relative to the totalout-and-back light diffraction efficiency of the sixteenth holographicfacet;

FIG. 10E2 sets forth a set of graphs plotting, as a function of themodulation-depth (i.e. modulation-index) with a fixed thickness gelatin,(1) the light diffraction efficiency of the sixteenth holographicscanning facet to S-polarized outgoing light rays incident on thesixteenth holographic facet, (2) the light diffraction efficiency of thesixteenth holographic scanning facet to P-polarized outgoing light raysincident on the sixteenth holographic facet, and (3) the totalout-and-back light diffraction efficiency of the sixteenth holographicscanning facet to S-polarized outgoing light rays incident on thesixteenth holographic facet, in order to ultimately compute the totalout-and-back light diffraction efficiency of the sixteenth holographicscanning facet relative to the total out-and-back light diffractionefficiency of itself (i.e. the sixteenth holographic scanning facet);

FIG. 10F is a schematic diagram illustrating the path travelled by thelight rays associated with an incident laser beam being initiallydiffracted by a rotating holographic facet towards a bar code symbol,then returning light rays reflected therefrom being diffracted again bythe same holographic facet towards the light focusing parabolic mirror,and finally the focused light rays being transmitted through the sameholographic scanning facet towards the polarized photodetector withoutsubstantial diffraction;

FIGS. 10F1 and 10F2 set forth geometrical optics models of the processof a laser beam propagating through a holographic scanning facet on therotating scanning disc shown in FIG. 10F, which are used during the discdesign process to compute the normalized total out-and-back lightdiffraction efficiency of each holographic scanning facet in theholographic scanning disc of the present invention, when across-polarizer is used in the holographic laser scanner;

FIG. 10G sets forth a set of parameters used to represent thegeometrical optics models of FIGS. 10F1 and 10F2;

FIG. 10G1 sets forth a set of initial (i.e. assumed) values forparticular parameters used to represent the geometrical optics models ofFIGS. 10F1 and 10F2;

FIG. 10H1 sets forth a set of mathematical equations describingstructural and functional relationships among particular parameters ofthe geometrical optics model of FIGS. 10F1 and 10F2;

FIG. 10H2 sets forth a set of equations defining (1) the lightdiffraction efficiency of the i-th holographic scanning facet of FIG.10F to S-polarized outgoing light rays incident thereto, (2) the lightdiffraction efficiency of the i-th holographic scanning facet toP-polarized outgoing light rays incident thereto, and (3) the totalout-and-back light diffraction efficiency of the i-th holographicscanning facet to S-polarized outgoing light rays incident thereto, eachbeing expressed as a function of the modulation-depth (i.e.modulation-index) within a fixed thickness gelatin;

FIG. 10H3 sets forth a set of equations used to calculate both Fresnellosses and transmission of P and S polarized light rays passing throughthe holographic scanning facets on the scanning disc, for use in thelight diffraction efficiency expression set forth in FIG. 10H2;

FIG. 10I1 sets forth a set of graphs plotting, as a function of theindex modulation-depth (i.e. modulation-index) with a fixed thicknessgelatin, (1) the light diffraction efficiency of the first holographicscanning facet to S-polarized outgoing light rays incident thereto, (2)the light diffraction efficiency of the first holographic scanning facetto P-polarized outgoing light rays incident thereto, and (3) the totalout-and-back light diffraction efficiency of the first holographicscanning facet to S-polarized outgoing light rays incident thereto, inorder to ultimately compute the total out-and-back light diffractionefficiency of the first holographic scanning facet relative to the totalout-and-back light diffraction efficiency of the sixteenth holographicscanning facet;

FIG. 10I2 sets forth a set of graphs plotting, as a function of theindex modulation-depth (i.e. modulation-index) with a fixed thicknessgelatin, (1) the light diffraction efficiency of the sixteenthholographic scanning facet to S-polarized outgoing light rays incidentthereto, (2) the light diffraction efficiency of the sixteenthholographic scanning facet to P-polarized outgoing light rays incidentthereto, and (3) the total out-and-back light diffraction efficiency ofthe sixteenth holographic scanning facet to S-polarized outgoing lightrays incident thereto, in order to ultimately compute the totalout-and-back light diffraction efficiency of the sixteenth holographicscanning facet relative to itself (i.e. H₁₆(Δn)=1);

FIG. 10J is a geometrical optics model illustrating the Lambertian lightcollecting efficiency of the i-th holographic scanning facet on thescanning disc of the present invention;

FIG. 10K sets forth a description of the parameters associated with thegeometrical optics model of FIG. 10J;

FIG. 10L sets forth a table of the initial (i.e. assumed) values forparticular parameters associated with the geometrical optics model ofFIG. 10J;

FIG. 10L1 sets forth a set of equations describing the relationshipsamong the particular parameters in the geometrical optics model of FIG.10J;

FIGS. 11A through 11C set forth a flow chart describing, in detail, thesteps of the method used to design the holographic scanning disc hereofaccording to the first illustrative embodiment of the present invention;

FIG. 12 is graphical plot of the light diffraction efficiency of anexemplary holographic scanning facet of the scanning disc of FIG. 3 to Spolarized light incident thereto, as a function of the refractive indexmodulation Δn_(i) (i.e. E_(S)(Δn_(i))), and the light diffractionefficiency of the inner light-collecting portion of the exemplaryholographic scanning facet to P polarized light incident thereto, as afunction of modulation index Δn_(i) (i.e. E_(P)(Δn_(i))), clearlyshowing that such light diffraction efficiencies E_(S)(Δn_(i)) andE_(P)(Δn_(i)) do not have peak values at the same value of modulationindex Δn_(i) and thus cannot be optimized using the same modulationindex Δn_(i) over the entire surface area of the scanning facet;

FIG. 12A is a schematic diagram of the holographic scanning disc of analternative embodiment of the present invention hereof, in which theouter beam-steering portion of each holographic scanning facet on thescanning disc has a light diffraction efficiency E_(S)(Δn_(i)) which isoptimized for an incident laser beam of a first (e.g. S) polarizationstate by selection of a first optimal modulation index Δn₁, whereas theinner light-collecting portion of the holographic scanning facet has alight diffraction efficiency E_(P)(Δn_(i)) which is optimized forreflected laser light of a second (i.e. P) polarization state orthogonalto the first polarization state by selection of a second optimalmodulation index Δn₂;

FIGS. 12B1 through 12B3 provide a flow chart describing, in detail, thesteps of the method used to design the holographic scanning disc shownin FIG. 12A;

FIG. 12C is a mathematical expression for the effective relative lightdiffraction efficiency for facet No. 1 on the scanning disc of FIG. 12A;

FIG. 13 is a geometrical optics model of a holographic recording systemwhich can be used to construct each holographic scanning facet of thescanning disc of the present invention, using the constructionparameters determined from the parameter conversion process illustratedin FIGS. 28A1 through 28D;

FIG. 14 is a partially cut-away, side cross-sectional view of onescanning channel of the laser scanning system of the first illustrativeembodiment of the present invention, showing the scanning window of thescanner housing, the holographic scanning disc rotatably supported bythe motor, the laser beam production module associated with theillustrated scanning channel, its beam folding mirror, parabolic lightcollecting mirror, and photodetector;

FIG. 14A is a partially cut-away, side cross-sectional view of onescanning channel of the laser scanning system of the first illustrativeembodiment of the present invention, showing computer-generatedschematic indications of both the outgoing and incoming optical pathstraversed by laser light produced and detected during the operation ofthe system;

FIG. 15 is a plan view of the laser beam production module according tothe first illustrative embodiment of the present invention comprising avisible laser diode (VLD), an aspherical collimating lens supportedwithin gimbal-like adjustable mounting assembly, and a prism mountedupon a rotatably adjustable platform, and a beam direction changingmirror and a holographic light diffractive grating supported above theoptical bench of module;

FIG. 15A is a plan view of the laser beam production module of FIG. 15,with the holographic light diffractive grating and planar mirror removedfrom the optical bench thereof;

FIG. 15B is a plan view of the optical bench of the laser beamproduction module of FIG. 15;

FIG. 15C is a side view of the optical bench of the laser beamproduction module of FIG. 15;

FIG. 15D1 is a side view of the prism support platform of the laser beamproduction module of FIG. 15;

FIG. 15D2 is a plan view of the prism support platform of the laser beamproduction module of FIG. 15;

FIG. 15E1 is a plan view of the VLD/lens mount pivot plate of the laserbeam production module of FIG. 15;

FIG. 15E2 is a side view of the VLD/lens mount pivot plate of the laserbeam production module of FIG. 15;

FIG. 15F1 is a plan view of the VLD/lens mounting bracket (i.e. yoke) ofthe laser beam production module of FIG. 15;

FIG. 15F2 is a side view of the VLD/lens mounting bracket of the laserbeam production module of FIG. 15;

FIG. 15G1 is a cross-sectional view of the VLD/lens mounting tube of thelaser beam production module of FIG. 15;

FIG. 15G2 is an axial view of the VLD/lens mounting tube of the laserbeam production module of FIG. 15;

FIG. 15H1 is an axial view of the lens barrel of the laser beamproduction module of FIG. 15;

FIG. 15H2 is a cross-sectional view of the lens barrel of the laser beamproduction module of FIG. 15;

FIG. 15I1 is a plan view of the prism of the laser beam productionmodule of FIG. 15;

FIG. 15I2 is a side view of the prism of the laser beam productionmodule of FIG. 15;

FIG. 15J is a plan view of the planar beam folding mirror of the laserbeam production module of FIG. 15;

FIG. 15K is a plan view of the holographic light diffractive grating(i.e. plate) of the laser beam production module of FIG. 15;

FIG. 16 is a flow chart illustrating the steps of the method used todesign the laser beam production module of the first illustrativeembodiment of FIG. 15A, using the module components shown in FIGS. 15Bthrough 15K;

FIG. 17A is a geometrical optics model of a holographic lightdiffractive grating illuminated with a laser beam produced from aconventional visible laser diode (VLD);

FIG. 17B is a set of parameters used to construct the geometrical opticsmodel of the laser beam being diffracted by the holographic lightdiffractive grating, as shown in FIG. 17A;

FIG. 17B1 is a set of assumed values for particular parameters used toconstruct the geometrical optics model of FIG. 17A;

FIG. 17C is a set of equations describing functional relationships amongcertain of the parameters of the geometrical optics model of FIG. 17A;

FIG. 17D is a graphical plot of the diffraction angle of an outgoinglaser beam versus the wavelength of the incident laser beam, showing thestrong functional dependence of the outgoing diffraction angle on thewavelength of the incident laser beam;

FIG. 18A is a geometrical optics model of the holographic optical systemformed by each holographic scanning facet on the scanning disc and theholographic light diffractive grating in the laser beam producing moduleof the first illustrative embodiment used to substantially decrease thefunctional dependence of the wavelength of an incident laser beam uponthe diffraction angle of the outgoing laser beam from the scanning disc;

FIG. 18B is a set of parameters used to mathematically represent thegeometrical optics model shown in FIG. 18A;

FIG. 18B1 is a set of assumed values for particular parameters in thegeometrical optics model of FIG. 18A;

FIG. 18C is a set of equations describing the relationships amongparticular parameters in the geometrical optics model of FIG. 18A;

FIG. 18D is a graphical plot of diffraction angle of the outgoing laserbeam versus the wavelength of the incident laser beam, for diffractionangles about the center portion of the diffraction angle range, showingthe substantial independence of the angle of diffraction of the outgoinglaser beam on the wavelength of the incident laser beam as a result ofthe optical arrangement of the present invention;

FIGS. 19A and 19B provide a geometrical optics model for an exemplaryholographic scanning facet, showing the various parameters used duringboth construction and reconstruction processes, and conversion from thereconstruction wavelength to the construction wavelength;

FIGS. 19C, 19D1, 19D2 and 19E are a set of given parameters, a set ofequations, and a resultant set of numbers, respectively, that determinethe hologram construction parameters at a second construction-laserwavelength given the desired hologram performance parameters at a firstscanner-laser wavelength.

FIG. 19F is a geometrical optics model of a system used for constructingthe holographic scanning facets using the construction parametersdetermined using the design process of the present invention;

FIG. 20 is a schematic representation of a laser diode, showing theinherent cause of astigmatic difference in visible laser diodes,attributable to the difference in location of the effective sources ofthe perpendicular and parallel laser beams emitting from the diodejunction;

FIG. 20A is a schematic diagram of the optical system used in the laserbeam production module of FIG. 15A, for simultaneously circularizing thelaser beam and eliminating astigmatism in the laser beam beyond the beamcircularizing prism;

FIGS. 20B1, 20B2 and 20B3 provide a geometrical optics model of theoptical system of FIG. 20A;

FIG. 20C is a set of parameters used to represent the geometrical opticsmodel of FIGS. 20B1 through 20B3;

FIG. 20C1 is set of assumed values for parameters in the geometricaloptics model of FIGS. 20B1 through 20B3;

FIGS. 20D and 20D1 set forth a set of equations describing functionalrelationships among particular parameters in the geometrical opticsmodel of FIGS. 20B1 through 20B3;

FIG. 20E is a graphical plot of the distances of the P and S sourceimages (i.e. L_(S2) and L_(P2)) projected by the aspheric collimatinglens in the laser beam production module of FIG. 15A, as a function ofthe distance from the focal point of the aspheric collimating lens tothe S-beam source (i.e. d), showing the value of distance (d) at whichthe P and S sources images converge and astigmatism is reduced to zero;

FIG. 21A is a schematic diagram of an optical system used in aligningthe components of the first optical system in the laser beam productionmodule of the first illustrative embodiment, so that astigmatism beyondthe prism is reduced to zero;

FIG. 21B is a flow chart indicating the steps of a procedure used toalign the components of the first optical system in the laser beamproduction module of the first illustrative embodiment, so that adesired beam aspect ratio (i.e. “1” for circular beam cross-section) isachieved and astigmatism in the laser beam beyond the second surface ofthe prism is reduced to zero;

FIG. 21C is a flow chart for a generalized parameter adjustmenttechnique of the present invention;

FIGS. 21C1, 21C2 and 21C3, taken together, provide a flow chartdescribing a specific procedure for assembling the components of thelaser beam production module of the first illustrative embodiment, andalso for configuring the geometrical and optical parameters thereof inaccordance with the principles of the present invention; and

FIG. 21D is an elevated cross-sectional view of the first and secondoptical systems of the laser beam production module of the firstillustrative embodiment shown coupled together with their geometricaland optical parameters configured to achieve beam dispersionminimization, beam aspect-ratio control, and astigmatism elimination;

FIG. 22 is a partially cut-away, side cross-sectional view of onescanning channel of the laser scanning system of the second illustrativeembodiment, showing the scanning window of the scanner housing, theholographic scanning disc rotatably supported by the motor, the laserbeam production module of the second illustrative embodiment, itsassociated beam folding mirror, parabolic light collecting mirror, andphotodetector;

FIG. 23 is an elevated side view of the laser beam production module ofthe second illustrative embodiment of the present invention, installedupon the optical bench of the laser scanner of the illustrativeembodiment with its first and second optical systems coupled together;

FIG. 23A is a plan view of the laser beam production module of thesecond illustrative embodiment of the present invention, shown with itsbeam folding mirror and dual-function holographic light diffractivegrating removed from the optical bench of the laser beam productionmodule;

FIG. 24 is a flow chart illustrating the steps involved in designing thelaser beam production module of FIG. 23 according to the design methodof the present invention;

FIG. 25A is a geometrical optics model of the first optical system (i.e.a holographic scanning facet and holographic light diffraction grating)associated with the laser production module of the second illustrativeembodiment;

FIG. 25B is a set of parameters used to represent the geometrical opticsmodel of FIG. 25A;

FIG. 25B1 is a set of assumed values for parameters in the geometricaloptics model of FIG. 25A;

FIG. 25C is a set of mathematical expressions describing relationshipsamong particular parameters in the geometrical optics model of FIG. 25A;

FIG. 25D provides two plots showing the relationship between (i) thebeam incidence angle θ_(i1D) upon the dual-function diffraction gratingand the orientation (i.e. tilt angle ρ) of the diffraction gratingrelative to the holographic scanning disc which provides zero dispersionand (ii) the beam incidence angle θ_(i1M) upon the diffraction gratingand the tilt angle ρ of the diffraction grating relative to theholographic scanning disc which provides a desired beam aspect-ratio,wherein the intersection point of these functional plots proves thatzero beam dispersion and a desired beam expansion ratio can be achievedby proper selection of tilt angle ρ;

FIG. 25E is a set of construction parameters for constructing thedual-function HOE of the illustrative embodiment of the presentinvention;

FIG. 26 is a geometrical optics model of the second optical system ofthe laser beam production module of the second illustrative embodiment,constructed by the Beam Dispersion Analyzer of the present invention inorder to determine the performance of this system;

FIG. 27A is a set of parameters used to represent the geometrical opticsmodel of FIG. 26;

FIG. 27B is a set of assumed values for parameters in the geometricaloptics model of FIG. 26;

FIG. 27C is a set of mathematical expressions describing relationshipsamong particular parameters of the geometrical optics model of FIG. 26;

FIG. 27D is a plot showing the relationship that exists between (i) thediffraction angle at the holographic disc of an incident laser beamproduced from a visible laser diode and (ii) the wavelength thereof whenusing the first optical system of FIG. 23 to precondition the laser beamprior to its passage through the holographic disc of the holographicscanning system hereof;

FIG. 27D1 is a table of values associated with the graphical plot ofFIG. 27D;

FIGS. 28A1 and 28A2 provide a geometrical optics model of the process ofchanging construction beam angles for a change in wavelength betweenconstruction and reconstruction;

FIG. 28B is a set of parameters used to represent the geometric opticsmodel of FIGS. 28A1 and 28A2 including a set of assumed values forparameters in the geometric optics model thereof;

FIGS. 28C1, 28C2 and 28D set forth a set of given parameters, a set ofequations, and a resultant set of numbers, that determine the hologramconstruction parameters at a second construction-laser wavelength giventhe desired hologram performance parameters at a first scanner-laserwavelength;

FIG. 29 is a schematic diagram of a holographic recording system forconstructing the dual-function diffraction grating, using theconstruction parameters determined from the parameter conversion processof FIGS. 28B and 28C;

FIGS. 30A, 30A1, and 30A2 and 30A3 provide a geometrical optics model ofthe second optical system of the laser beam production module of thesecond illustrative embodiment shown in FIG. 23;

FIG. 30B is a set of parameters used to represent the geometrical opticsmodel of FIG. 30A;

FIG. 30B1 is a set of assumed values for certain fixed parameters usedto construct the geometrical optics model of FIG. 30B;

FIGS. 30C1 and 30C2 set forth a set of mathematical equations describingrelationships among particular parameters of the geometrical opticsmodel of FIG. 30A;

FIG. 30D is a graphical plot of the distances of the P and S sourceimages (i.e. L_(S2) and L_(P2)) projected by the aspheric collimatinglens in the laser beam production module of the second illustrativeembodiment, and the distance from the focal point of the collimatinglens to the S-beam source (i.e. d), showing that there exists a value ofdistance d at which the P and S source images L_(S2) and L_(P2) convergeand astigmatism is reduced to zero;

FIGS. 31A1 and 31A2 provide a schematic diagram of the optical systemused in aligning the components of the second optical system in thelaser beam production module of the first illustrative embodiment, sothat astigmatism beyond the dual-function diffractive grating is reducedto zero;

FIG. 31B is a flow chart indicating the procedural steps used to alignthe components of the second optical system in the laser beam productionmodule of FIG. 23 so that astigmatism beyond the dual-function HOE isreduced to zero;

FIGS. 31C1 and 31C2 provide a flow chart describing a procedure forassembling the components of the laser beam production module of thesecond illustrative embodiment and configuring the geometrical andoptical parameters thereof in accordance with the principles of thepresent invention;

FIG. 31D is an elevated side view of the first and second opticalsystems of the laser beam production module of FIG. 23 shown coupledtogether and mounted on the optical bench of the holographic scannerhereof;

FIG. 32 is a partially cut-away, side cross-sectional view of onescanning channel of the laser scanning system of the second illustrativeembodiment of the present invention, showing the light detectionsubsystem of the first illustrative embodiment comprising theholographic scanning disc rotatably supported by the motor, the laserbeam production module associated with the illustrated scanning channel,its beam folding mirror, parabolic light focusing mirror, andphotodetector;

FIGS. 33A, 33B and 33C provide a flow chart describing a method ofdesigning a light collection and detection subsystem for a holographicscanner according to the present invention;

FIG. 34 is a geometrical model of the holographic scanner under designprior to the specification of the parabolic mirror and photodetectors;

FIGS. 35A1 and 35A2 provide a geometrical optics model of the lightdetection subsystem shown in FIG. 32, which does not usecross-polarizers;

FIG. 35B is a set of parameters used to represent the optics model ofFIGS. 35A1 and 35A2;

FIG. 35B1 is a set of assumed values for parameters used in the opticsmodel of FIGS. 35A1 and 35A2;

FIGS. 35C1 and 35C2 set forth a set of mathematical expressionsdescribing relations among particular parameters of the geometricaloptics model of FIGS. 35A1 and 35A2;

FIG. 35D1 provides a plot of the normalized “average” light diffractionefficiency of the holographic scanning facet No. 1 on the scanning discas a function of the amount of angular degrees off Bragg (i.e. δ_(e)),where normalized is with respect to the peak diffraction efficiency offacet No. 1 at the Bragg angle.

FIG. 35D2 provides a plot of the normalized “average” light diffractionefficiency of the 16-th holographic scanning facet on the scanning discas a function of the amount of angular degrees off Bragg (i.e. δ_(e))where normalized is with respect the peak diffraction efficiency offacet No. 16 at the Bragg angle.

FIG. 36 is a partially cut-away, side cross-sectional view of onescanning channel of the laser scanning system hereof, showing the lightdetection subsystem of the second illustrative embodiment comprising theholographic scanning disc rotatably supported by the motor, the laserbeam production module associated with the illustrated scanning channel,its beam folding mirror, parabolic light focusing mirror, photodetector,and a cross S polarizing filter disposed in front of the photodetector;

FIG. 37A is a set of parameters used to represent the optics model ofthe subsystem of FIG. 36 in which an S polarizing filter is placedbefore the photodetector, and the geometrical optics model thereof has asimilar structure to the geometrical optics model shown in FIGS. 35A1and 35A2 for the subsystem not employing cross-polarizers;

FIG. 37A1 is a set of assumed values for parameters used in the opticsmodel of the subsystem of FIG. 36;

FIGS. 37B sets forth a set of mathematical expressions describingrelations among particular parameters of the geometrical optics model ofthe subsystem of FIG. 36;

FIG. 37C1 provides a plot of the normalized light diffraction efficiencyof holographic scanning facet No. 1 on the scanning disc to S Polarizedlight, expressed as a function of the amount of angular degrees offBragg (i.e. δ_(e)), where normalization is with respect to the peakdiffraction efficiency of facet No. 1 at the Bragg angle;

FIG. 37C2 provides a plot of the normalized light diffraction efficiencyof the 16-th holographic scanning facet on the scanning disc to Spolarized light, expressed as a function of the amount of angulardegrees off Bragg (i.e. δ_(e)) where normalization is with respect tothe peak diffraction efficiency of facet No. 16 at the Bragg angle;

FIG. 38A is a set of parameters used to represent the optics model ofthe subsystem of FIG. 36, in which a S polarizing filter is placedbefore the photodetector, and the geometrical optics model thereof has asimilar structure to the geometrical optics model shown in FIGS. 35A1and 35A2 for the subsystem not employing cross-polarizers;

FIG. 38A1 is a set of assumed values for parameters used in the opticsmodel of the subsystem of FIG. 36;

FIG. 38B1 and 38B2 sets forth a set of mathematical expressionsdescribing relations among particular parameters of the geometricaloptics model of the subsystem of FIG. 36, where a S polarizer is used;

FIG. 38C1 provides a plot of the normalized light diffraction efficiencyof holographic scanning facet No. 1 on the scanning disc to P Polarizedlight, expressed as a function of the amount of angular degrees offBragg (i.e. δ_(e)), where normalization is with respect to the peakdiffraction efficiency of facet No. 1 at the Bragg angle;

FIG. 38C2 provides a plot of the normalized light diffraction efficiencyof the 16-th holographic scanning facet on the scanning disc to Ppolarized light, expressed as a function of the amount of angulardegrees off Bragg (i.e. δ_(e)) where normalization is with respect tothe peak diffraction efficiency of facet No. 16 at the Bragg angle;

FIG. 39 is a ray optics diagram showing the paths of the innermost andoutermost light rays collected by a holographic scanning facet on thescanning disc associated with the light detection subsystem of thepresent invention;

FIG. 40A is a plan view of a 3-D geometrical model of scanning discwithin the laser scanner of the present invention, illustrating thefirst step of the method used to determine the first widthwise boundaryof the parabolic light collecting surface patch being designed for usein the light detecting subsystem of the system hereof;

FIG. 40B is a plan view of a 3-D geometrical model of scanning discwithin the laser scanner of the present invention, illustrating thesecond step of the method used to determine the second widthwiseboundary of the parabolic light collecting surface patch being designedfor use in the light detecting subsystem of the system hereof;

FIG. 41 is a partially cut-away, side cross-sectional view of onescanning channel of the laser scanning system of the fifth illustrativeembodiment of the present invention, showing the scanning window of thescanner housing, the transmission-type volume holographic scanning discrotatably supported by the motor, the laser beam production moduleassociated with the illustrated scanning channel, its beam foldingmirror, volume-reflection type holographic light focusing element, andphotodetector;

FIG. 42 is a partially cut-away, side cross-sectional view of onescanning channel of the laser scanning system of the sixth illustrativeembodiment of the present invention, showing the transmission-typevolume holographic scanning disc rotatably supported by the motor, thelaser beam production module associated with the illustrated scanningchannel, its beam folding mirror, and a single light folding mirror,light focusing optics and photodetector disposed beneath the scanningdisc;

FIGS. 43A and 43B provide partially cut-away, side cross-sectional viewsof one scanning channel of the laser scanning system of the seventhillustrative embodiment of the present invention, showing thetransmission-type volume holographic scanning disc rotatably supportedby the motor, the laser beam production module associated with theillustrated scanning channel, its beam folding mirror, and dual lightfolding mirrors, light focusing optics and photodetector disposedbeneath the scanning disc;

FIG. 44 is a partially cut-away, side cross-sectional view of onescanning channel of the laser scanning system of the eighth illustrativeembodiment of the present invention, showing the reflection-type volumeholographic scanning disc rotatably supported by the motor, the laserbeam production module associated with the illustrated scanning channel,its beam folding mirror, and a volume-transmission type holographiclight focusing element and photodetector disposed above the scanningdisc;

FIGS. 45A and 45B are perspective schematic views of a code symbolscanning system, in which the holographic laser scanner of the presentinvention is used to detect the presence of code symbols within itsscanning volume, and a high-speed laser scanner with variable focaldistance is used to scan the region in which the detected code symbolresides to collect high-resolution scan data for use in decodeprocessing;

FIG. 46 is a perspective view of an automatic, hand-supportableholographic laser scanning device constructed in accordance with theprinciples of the present invention;

FIG. 47 is a schematic representation of a automatic, hand-supportableholographic scanning device constructed in accordance with the presentinvention, and which produces a two-dimensional raster-type laserscanning pattern within its 3-D scanning volume; and

FIG. 48 is a schematic representation of an automatic holographic laserscanning engine of the present invention, shown mounted on the back of auser's hand for hands-free scanning applications.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS OF THE PRESENTINVENTION

Referring to the figures in the accompanying Drawings, the variousillustrative embodiments of the holographic laser scanner of the presentinvention will be described in great detail.

In the illustrative embodiments, the apparatus of the present inventionis realized in the form of an automatic code symbol reading systemhaving a high-speed holographic laser scanning mechanism as well as ascan data processor for decode processing scan data signals producedthereby. However, for the sake of convenience of expression, the term“holographic laser scanner” shall be used hereinafter to denote the barcode symbol reading system which employs the holographic laser scanningmechanism of the present invention.

The Holographic Laser Scanning System Employing a Transmission-VolumeType Holographic Laser Scanning Disc

As illustrated in FIGS. 1A, 1B and 1C, the holographic laser scanner ofthe present invention 1 can be used in a diverse variety of code symbolscanning applications. In FIG. 1A, the holographic laser scanner isinstalled in a warehouse and is used to read bar code symbols 2 onpackages 3 for sorting and routing purposes. In FIG. 1B, the holographiclaser scanner is installed above the doorway of a storage warehouse andis used to read bar code symbols on packages being loaded in as well asunloaded from the warehouse, as part of an automated inventory controloperation. In FIG. 1C, the holographic laser scanner is shown installedabove the doorway of a storage container parked against a loading dock,and is used to read bar code symbols on packages being loaded in orunloaded from the container, also as part of an automated inventorycontrol operation.

In FIGS. 2A through 2E, the holographic scanning system 1 is shown withits compact housing enclosure 4 removed from its base 5 which functionsas an optical bench for its various optical and electro-opticalcomponents. In the illustrative embodiment, the total height of thescanner housing is 6.96 inches, with width and length dimensions of 12.0and 13.7 inches, respectively, to provide a total internal housingvolume (“scanner volume”) V_(housing) of about 1144 cubic inches with ascanner housing depth of 6.96 inches. As will be described in greaterdetail below, the total three-dimensional scanning volume produced bythis ultra-compact housing is 15043.6 cubic inches with a scanning fielddepth of 30.0 inches. Importantly, the resolution of the bar code symbolthat the scanning pattern of the illustrative embodiment can resolve atany location within the specified three-dimensional laser scanningvolume V_(scanning) is on the order of about 0.017 inches minimumelement width. In the illustrative embodiment (the figure of meritV_(scanning)/V_(housing)=13.15. As will become apparent hereinafter,using the design principles and methods of the present inventiondisclosed herein, the figure of merit V_(scanning)/V_(housing) can bemaximized under a various range of conditions.

As shown in FIG. 2A, the holographic scanning system of the illustrativeembodiment comprises three laser scanning stations 6A, 6B and 6C,symmetrically arranged about a holographic scanning disc 7. As bestillustrated in FIGS. 2B and 3, the holographic scanning disc 7 comprisestwo glass plates 8A and 8B, between which are supported a plurality ofspecially designed holographic optical elements (HOEs), referred tohereinafter as “holographic scanning facets” or “holographic facets”. Inthe illustrative embodiments, each holographic facet 9 is realized as avolume transmission-type light diffraction hologram having a slantedfringe structure having variations in spatial frequency to provide acharacteristic focal length f_(i). The light diffraction efficiency ofsuch volume light diffraction holograms, as a function of incidenceangle A_(i), modulation depth Δn_(i), or recording media losses, isdescribed in great detail in the celebrated paper entitled “Coupled WaveTheory for Thick Hologram Gratings” by Herwig Kogelnik, published in TheBell System Technical Journal (BSTJ), Volume. 8, Number 9, at Pages2909-2947, in November 1969, incorporated herein by reference in itsentirety.

In a conventional manner, the glass support plates 8A and 8B formingpart of the holographic scanning disc hereof are mounted to a supporthub 10. In turn, the support hub is mounted to the shaft of ahigh-speed, electric motor 11. The other principal subcomponents of eachlaser scanning station are a laser beam production module 12A (12B,12C), a planar beam folding mirror 13A (13B, 13C), a parabolic lightfocusing element (e.g. mirror or volume reflection hologram) 14A (14B,14C), a photodetector 15A (15B, 15C) with an optional cross-polarizingfilter element 16A (16B, 16C) disposed thereacross, an analog scan datasignal processing board 17A (17B, 17C), and a digital scan data signalprocessing board 18A (18B, 18C). For purposes of simplicity ofdescription, when describing the laser scanning stations of the presentinvention, reference will be made to station 6A. It is understood,however, the stations 6B and 6C have similar structure and operate insubstantially the same manner as Station 6A.

The function of each laser beam production module is to cooperate withthe holographic scanning disc and produce from its internal visiblelaser diode (VLD), a laser beam with desired beam cross-sectionalcharacteristics (e.g. having the beam aspect ratio of an ellipse orcircle) and being essentially free of astigmatism and beam-dispersionthat is otherwise associated with a laser beam directly transmitted froma VLD through a rotating holographic scanning facet during laser beamscanning operations. When an incident laser beam from the VLD passesthrough a particular holographic scanning facet on the rotating scanningdisc, it is diffracted in a prespecified “outgoing” direction (i.e. atan angle of diffraction B_(i)) determined during the holographic discdesign process of the present invention. The function of the beamfolding mirror associated with each scanning station is to change (i.e.fold) the direction of the outgoing diffracted laser beam from itsoutgoing direction, into the direction required to generate itscorresponding laser scanning plane. Notably, when the produced laserscanning plane is intersected by a planar surface (e.g. bearing a barcode symbol), a linear scanline is projected on the intersected surface,as illustrated in FIG. 5. The angular dimensions of each resultingscanning plane are determined by the Scan Angle, θ_(Si), associated withthe geometry of the scanning facet and the Scan Angle MultiplicationFactor, M_(i), associated therewith, which will be discussed in greaterdetail hereinafter. When a bar code symbol is scanned by any one of thelaser scanning planes, the incident laser light is scattered (accordingto Lambert's Law for diffuse reflective surfaces). A portion of thislaser light is reflected back along the outgoing ray path, off the beamfolding mirror and thereafter passes through the same holographicscanning facet that generated the corresponding scanning plane onlyT_(transit)=2−f_(i)/c seconds before, where c is the speed of light. Asthe reflected laser light passes through the holographic scanning faceton its return path towards the parabolic mirror beneath the scanningdisc, the incoming light rays enter the holographic scanning facet closeto the Bragg angle thereof (i.e. B_(i)) and thus (once again) arestrongly diffracted towards the parabolic mirror along its optical axis.The parabolic mirror, in turn, focuses these collected light rays andredirects the same through the holographic scanning facet at anglessufficiently far off the Bragg angle (i.e. A_(i)) so that they aretransmitted therethrough towards the photodetector with minimal lossesdue to internal diffraction within the holographic facet. A novel methodof designing the light detection subsystem of the present invention willbe described in great detail hereinafter for various types ofholographic scanning discs and light polarization techniques.

As best shown in FIG. 3, the holographic facets on the holographicscanning disc of the present invention are arranged on the surfacethereof in a manner which utilizes substantially all of the lightcollecting surface area provided between the outer radius of thescanning disc, r_(outer), and the inner radius thereof, r_(inner). Inthe illustrative embodiment, sixteen holographic scanning facets areused in conjunction with the three independent laser beam sources, toprovide an omni-directional laser scanning pattern consisting offorty-eight (48) laser scanning planes cyclically generated at a rate inexcess of 56 times per second. It is understood, however, this numberwill vary from embodiment to embodiment of the present invention andthus shall not form a limitation thereof. As will be described ingreater detail hereinafter, the geometry of each holographic facet hasbeen designed so that (1) each of the sixteen holographic facetssupported thereon has substantially the same (i.e. equal) Lambertianlight collecting efficiency, independent of its focal length, and (2)the collective surface area of all of the holographic facets occupies(i.e. uses) all of the available light collecting surface area betweenthe outer radius and inner radius of the scanning disc. The advantage ofthis aspect of the present invention is that optical-based scan datasignals with maximum signal-to-noise (SNR) ratio are produced andcollected at the photodetector of each laser scanning station in thesystem. This, of course, implies higher performance and higher qualityscan data signals for signal processing.

As shown in FIG. 3, each holographic facet on the surface of thescanning disc is specified by a set of geometrical parameters, a set ofoptical parameters, and a set of holographic recording parameters. Thegeometrical parameters define various physical characteristics of thefacet in issue, such as the location of the facet on the disc specifiedby its preassigned facet number (e.g. i=1, 2, 3, . . . or 16), its lightcollecting surface Area_(i) (designed to exhibit a high diffractionefficiency to incoming light rays on Bragg), the Angle of the facetθ_(roti), the adjusted Rotation Angle of the facet θ′_(roti) actual scanangle of the facet θ_(Sweepi) (accounting for beam diameter d_(beam) andinterfaced gaps d_(gap)), and the surface boundaries SB_(i) occupied bythe holographic facet on the scanning disc, which typically will beirregular in shape by virtue of the optimized light collecting surfacearea of the holographic disc). The optical parameters associated witheach holographic facet include the wavelength λ at which the object beamis designed to be reconstructed, the angle of incidence of theholographic facet A_(i), the angle of diffraction thereof B_(i), itsscan angle multiplication factor M_(i), the focal length f_(i) of thefacet, etc. Unlike the other parameters associated with each facet, therecording parameters define the thickness, T, of the recording medium(e.g. dichromate gelatin) used during the recording of the holographicfacet, the average bulk index of refraction of the recording medium, andthe modulation depth (i.e. modulation-index) Δn_(i) associated withfringe structure formed in the recording medium. Collectively, theseparameters shall be referred to as “construction parameters”, as theyare required to construct the holographic facet with which they areassociated.

In the scanning system of the present invention, the principal functionof each holographic facet is to deflect an incident laser beam along aparticular path in 3-D space in order to generate a correspondingscanning plane within the 3-D laser scanning volume produced by thescanning system. Collectively, the complex of laser scanning planesproduced by the plurality of holographic facets in cooperation with thethree laser beam production modules, creates the highly confined 3-Dscanning pattern within the highly defined scanning volume of thescanning system.

As shown in FIG. 5, the holographic laser scanner of the illustrativeembodiment cyclically generates from its ultra compact scanner housing4, a complex three-dimensional laser scanning pattern consisting offorty-eight laser scanning planes, with four different focal planes.This implies that twelve different laser scanning planes are focused ineach of the four different focal planes within the 3-D scanning volume.As shown, each of these focal planes extend substantially parallel tothe scanning window of the holographic laser scanner and are located atdifferent distances from the scanning window. Thus when each one ofthese scanning planes is intersected by a planar object, such as acarton wall-surface, twelve laser scanning lines are projected onto itssurface, as best shown in FIG. 5. Greater details of the laser scanningpattern of the present invention will be described hereinbelow.

In FIG. 2B, one of the laser scanning stations in the holographicscanner is shown in greater detail. As illustrated in this figure, thebeam folding mirror associated with each laser scanning station, has asubstantially planar reflective surface 15 and is tangentially mountedadjacent the holographic scanning disc. In the illustrative embodiment,beam folding mirror 13A is supported in this position relative to thehousing base (i.e the optical bench) 5 using support legs 16A and 17Aand rear support bracket 18A. The angle of inclination of the (j-th beamfolding mirror relative to the normal to the holographic disc, φ, willbe specified in greater detail during the description of the scannerdesign process of the present invention. Notably, in order to minimizethe height of the holographic scanner housing designated as “h”, andthus design a truly ultra-compact holographic laser scanner, it isnecessary to minimize the height of each j-th beam folding mirrorrelative to housing base designated as “Y_(j)”. As will be described ingreat detail hereinafter, the design process of the present inventionprovides a way in which to determine the minimum height of the beamfolding mirrors Y_(j), given a prespecified laser scanning pattern,resolution, and holographic disc size, and thus provides a novel methodof designing a compact holographic laser scanner having physicaldimensions hitherto unattainable using prior art techniques. While thedesign method of the present invention is shown herein applied to acompact, transportable holographic laser scanner, it is readilyapplicable to hand-held hand-supportable as well as body mountableholographic laser scanners.

As shown in FIG. 2B, the laser beam production module associated witheach laser scanning station is mounted on the optical bench (i.e.housing base plate 5), immediately beneath its associated beam foldingmirror. Depending on which embodiment of the laser beam productionmodule is employed in the construction of the holographic laser scanner,the position of the laser beam production module may be different.However, it is preferred that the geometrical dimensions of its beamfolding mirrors in conjunction with the geometrical dimensions of itsholographic disc are the sole determinants of the width and lengthdimensions of the scanner housing, whereas the geometrical dimensions ofits beam folding mirrors and parabolic light focusing mirror beneath theholographic scanning disc are the sole determinants of the heightdimension of the scanner housing. This implies that when designing aholographic laser scanner according to the method of the presentinvention, the location of the laser beam production modules, the signalprocessing boards, the motor for rotating the holographic scanning disc,the photodetectors, the beam folding mirrors, the light detectionsubsystem, and all components other than the holographic scanning disc,do not impose constraints on the geometrical dimensions of the scannerhousing. In short, according to the design and construction principlesof the present invention, the above-described holographic scannercomponents can be mounted on the optical bench within the heightwise,widthwise and lengthwise boundary constraints set solely by thegeometrical dimensions of the holographic scanning disc, the beamfolding mirrors and the parabolic light collecting mirrors beneath theholographic disc. However, as will be shown during the detaileddescription of the scanner design method hereof, the geometricaldimensions of the laser scanning pattern within the 3-D scanning volumeV_(scanning) are what ultimately determine the heightwise, widthwise andlengthwise boundary constraints necessarily imposed on the geometricaldimensions of the holographic disc, the beam folding mirrors and theparabolic light collecting mirrors beneath the holographic scanningdisc. Thus, specifications for the laser scanning pattern to be realizedprovide fundamental constraints for the holographic scanner designprocess of the present invention.

As shown in FIGS. 2A through 2D, the three laser production modules 12A,12B and 12C are mounted on base plate 5, symmetrically about the axis ofrotation of the shaft of electric motor 11. During laser scanningoperations, these laser beam production modules produce threeindependent laser beams which are directed through the edge of theholographic disc at an angle of incidence A_(i), which, owing to thesymmetry of the laser scanning pattern of the illustrative embodiment,is the same for each laser scanning station (i.e. A_(i)=43.0 degrees forall values of i). The incident laser beams produced from the three laserbeam production modules 12A, 12B and 12C extend along the three centralreference planes 19A, 19B and 19C, each extending normal to the plane ofbase plate 5 and arranged 120° apart from its adjacent neighboringcentral planes, as best illustrated in FIG. 2D. While these centralreference planes are not real (i.e. are merely virtual), they will beuseful in describing the detailed geometrical structure of each laserscanning station in the holographic laser scanner of the presentinvention.

As shown in FIG. 2B, the photodetector of each laser scanning station ismounted along its central reference plane, above the holographic discand opposite its associated beam folding mirror so that it does notblock or otherwise interfere with the returning (i.e. incoming) laserlight rays reflecting off light reflective surfaces (e.g. productsurfaces, bar code symbols, etc) during laser scanning and lightcollecting operations. In the illustrative embodiment, the threephotodetectors 15A, 15B and 15C are supported in their respectivepositions by a photodetector support frame 20 which is stationarilymounted to the optical bench by way of vertically extending supportelements 21A, 21B and 21C. The electrical analog scan data signalproduced from each photodetector is processed in a conventional mannerby its analog scan data signal processing board which is also supportedupon photodetector support frame 20. Notably, the height of thephotodetector support board, referenced to the base plate (i.e. opticalbench), is chosen to be less than the minimum height that the beamfolding mirrors must extend above the holographic disc in order torealize the prespecified laser scanning pattern of the illustrativeembodiment. In practice, this height parameter is not selected (i.e.specified) until after the holographic disc has been completely designedaccording to the design process of the present invention, whilesatisfying the design constraints imposed on the disc design process. Aswill be explained in greater detail hereinafter, the use of aspreadsheet-type computer program to analytically model the geometricalstructure of both the laser scanning apparatus and the ray optics of thelaser beam scanning process, allows the designer to determine thegeometrical parameters associated with the holographic scanning facetson the disc which, given the specified maximum height of the beamfolding mirrors Y_(j), will produce the prespecified laser scanningpattern (including focal plane resolution) while maximizing the use ofthe available light collecting area on the holographic scanning disc.

As best shown in FIGS. 2B, 2C, 2D and 14, the parabolic light collectingmirror associated with each laser scanning station is disposed beneaththe holographic scanning disc, along the central reference planeassociated with the laser scanning station. While certainly notapparent, precise placement of the parabolic light collecting element(e.g. mirror) relative to the holographic facets on the scanning disc isa critical requirement for effective light detection by thephotodetector associated with each laser scanning station. Placement ofthe photodetector at the focal point of the parabolic light focusingmirror alone is not sufficient for optimal light detection in the lightdetection subsystem of the present invention. Careful analysis must beaccorded to the light diffraction efficiency of the holographic facetson the scanning disc and to the polarization state(s) of collected andfocused light rays being transmitted therethrough for detection. As willbecome more apparent hereinafter, the purpose of such light diffractionefficiency analysis ensures the realization of two important conditions,namely: (i) that substantially all of the incoming light rays reflectedoff an object (e.g. bar code symbol) and passing through the holographicfacet (producing the corresponding instant scanning beam) are collectedby the parabolic light collecting mirror; and (ii) that all of the lightrays collected by the parabolic light collecting mirror are focusedthrough the same holographic facet onto the photodetector associatedwith the station, with minimal loss associated with light diffractionand refractive scattering within the holographic facet. A detailedprocedure will be described hereinafter for designing and installing theparabolic light collecting mirror in order to satisfy the criticaloperating conditions above.

As shown in FIGS. 2A through 2D, the three digital scan data signalprocessing boards 18A, 18B and 18C are arranged in such a manner toreceive and provide for processing the analog scan data signals producedfrom analog scan data signal processing boards 17A, 17B and 17C,respectively. As best shown in FIGS. 2A and 2B, each digital scan datasignal processing board is mounted vertically between a pair of adjacentbeam folding mirrors, close to the tangential edge of the holographicdisc, within the scanner volume defined by the geometrical dimensions ofthe holographic disc and the beam folding mirrors. A central processingboard 21 is also mounted upon the base plate for processing signalsproduced from the digital scan data signal processing boards. Aconventional power supply board 22 is also mounted upon the base plate,within one of its extreme corners. The function of the digital scan datasignal processing boards, the central processing board, and the powersupply board will be described in greater detail in connection with thefunctional system diagram of FIG. 4. As shown, electrical cables areused to conduct electrical signals from each analog scan data signalprocessing board to its associated digital scan data signal processingboard, and from each digital scan data signal processing board to thecentral processing board. Regulated power supply voltages are providedto the central signal processing board 21 by way of an electricalharness (not shown), for distribution to the various electrical andelectro-optical devices requiring electrical power within theholographic laser scanner. In a conventional manner, electrical powerfrom a standard 120 Volt, 60 HZ, power supply is provided to the powersupply board by way of flexible electrical wiring (not shown). Symbolcharacter data produced from the central processing board is transmittedover a serial data transmission cable connected to a serial output (i.e.standard RS232) communications jack 23 installed through a wall in thescanner housing. This data can be transmitted to any host device 24 byway of a serial (or parallel) data communications cable, RF signaltransceiver, or other communication mechanism known in the art.

As shown in FIG. 2E, the scanner housing has three symmetricallyarranged light transmission apertures 25A, 25B and 25C formed in its topwall surface 26. Each of these light transmission apertures has asubstantially planar extent which is substantially parallel to thescanning disc rotatably supported upon the shaft of electric motor 11.In order to seal off the optical components of the scanning system fromdust, moisture and the like, a laser scanning window 26, preferablyfabricated from a high impact plastic material, is installed over eachlight transmission aperture using a rubber gasket and conventionalmounting techniques. In the illustrative embodiment, each laser scanningwindow 26 has spectrally-selective light transmission characteristicswhich, in conjunction with a spectrally-selective filter 27 installedbefore each photodetector within the housing, forms a narrow-bandspectral filtering subsystem that performs two different functions. Thefirst function of the narrow-band spectral filtering subsystem is totransmit only the optical wavelengths in the red region of the visiblespectrum in order to impart a reddish color or semi-transparentcharacter to the laser scanning window. This makes the internal opticalcomponents less visible and thus remarkably improves the externalappearance of the holographic laser scanning system. This feature alsomakes the holographic laser scanner less intimidating to customers atpoint-of-sale (POS) stations where it may be used. The second functionof the narrow-band spectral filtering subsystem is to transmit to thephotodetector for detection, only the narrow band of spectral componentscomprising the outgoing laser beam produced by the associated laser beamproduction module. Details regarding this optical filtering subsystemare disclosed in copending application Ser. No. 08/439,224, entitled“Laser Bar Code Symbol Scanner Employing Optical Filtering With NarrowBand-Pass Characteristics and Spatially Separated Optical FilterElements” filed on May 11, 1995, which is incorporated herein byreference in its entirety.

When using multiple laser beam sources in any holographic laser scanningsystem, the problem of “cross-talk” among the neighboring lightdetection subsystems typically arises and must be adequately resolved.The cause of the cross-talk problem is well known. It is due to the factthat the spectral components of one laser beam are detected by aneighboring photodetector. While certainly not apparent, the holographicscanning disc of the present invention has been designed so that lightrays produced from one laser beam (e.g. j=1) and reflected off a scannedcode symbol anywhere within the laser scanning volume V_(scanning) willfall incident upon the light collecting region of the scanning discassociated with a neighboring light detection subsystem in an off-Braggcondition. Consequently, the signal level of “neighboring” incoming scandata signals are virtually undetectable by each photodetector in theholographic laser scanner of the present invention. The opticalcharacteristics of the scanning facets on the scanning disc which makesthis feature possible will be described in greater detail hereinafterduring the description of the scanning disc design process hereof.

As best shown in FIG. 3, the holographic scanning disc of the presentinvention is unlike any other prior art laser scanning disc in twoimportant respects. Firstly, virtually all of the utilizable surfacearea of the scanning disc, defined between the outer edge of the supporthub 10 and the outer edge of the scanning disc, is occupied by thecollective surface area of all sixteen holographic scanning facets thathave been laid out over this defined region. Secondly, each holographicscanning facet has substantially the same Lambertian light collectionefficiency as all other scanning facets. Unlike conventional laserscanning discs, the geometry of each holographic facet on the scanningdisc of the present invention is apparently irregular, arbitrary andperhaps even fanciful to the eyes of onlookers. The fact is, however,that this is not the case. As will be described in greater detailhereinafter, the scanning disc design process hereof comprises two majorstages: a first, “analytical modelling stage” during which particularoptical and geometrical parameters are determined for each holographicfacet within a complex set of scanning system constraints; and a second,“holographic facet layout stage”, during which the scanning discdesigner lays out each holographic facet on the support disc so thatvirtually all of the available surface area thereon is utilized by theresulting layout. While the disc design method hereof allows certaingeometrical parameters associated with each designed holographic facetto be selected on the basis of discretion and judgement of the discdesigner (preferably using a computer-aided (CAD) tool) during theholographic facet layout stage, certain geometrical parameters, however,such as the total surface area of each facet Area_(i), its Scan SweepRotation (or Sweep Angle θ′_(rot)) and its inner radius r_(i) aredetermined during the analytical modelling stage by the geometricalstructure (e.g. its scanline length, focal plane, and relative positionin the scan pattern) associated with the corresponding laser scanlineP(i,j) produced by the holographic facet within a particular focal planeof the prespecified laser scanning pattern. Consequently, particularparameters determined during the analytical modelling stage of thedesign process operate as constraints upon the disc designer during thefacet layout stage of the process. Thus, the holographic facets realizedon the scanning disc of the present invention have particulargeometrical characteristics that are directly determined by geometricalproperties of the laser scanning pattern produced therefrom, as well asthe optical properties associated with the laser beam and theholographic facets realized on the scanning disc. This fact, whilepresently subtle, will become readily apparent during the detaileddescription of the holographic scanning disc design process of thepresent invention.

As shown in the system diagram of FIGS. 4A through 4C, the holographiclaser scanning system of the present invention comprises a number ofsystem components, many of which are realized on boards that have beenhereinbefore described. For sake of simplicity, it will be best todescribe these system components by describing the components realizedon each of the above-described boards, and thereafter describe theinterfaces and interaction therebetween.

In the illustrative embodiment, each analog scan data signal processingboard 17A, 17B, 17C has the following components mounted thereon: anassociated photodetector 17A (17B, 17C) (e.g. a silicon photocell) fordetection of analog scan data signals (as described); an analog signalprocessing circuit 35A (35B, 35C) for processing detected analog scandata signals; a 0-th diffraction order signal detector 36A (36B, 36C)for detecting the low-level, 0-th diffraction order signal produced fromeach holographic facet on the rotating scanning disc during scanneroperation; and associated signal processing circuitry 37A (37B, 37C) fordetecting a prespecified pulse in the optical signal produced by the0-th diffraction order signal detector and generating a synchronizingsignal S(t) containing a periodic pulse pattern. As will be describedbelow in greater detail, the function of the synchronizing signal S(t)is to indicate when a particular holographic facet (e.g. Facet No. i=1)produces its 0-th order optical signal, for purposes of linking detectedscan data signals with the particular holographic facets that generatedthem during the scanning process.

In the illustrative embodiment, each photodetector 17A, 17B and 17C isrealized as an opto-electronic device and each analog signal processingcircuit 35A (35B, 35C) aboard the analog signal processing board isrealized as an Application Specific Integrated Circuit (ASIC) chip.These chips are suitably mounted onto a small printed circuit (PC)board, along with electrical connectors which allow for interfacing withother boards within the scanner housing. With all of its componentsmounted thereon, each PC board is suitably fastened to the photodetectorsupport frame 20, along its respective central reference frame, as shownin FIG. 2B.

In a conventional manner, the optical scan data signal D₀ focused ontothe photodetector 16A (16B or 16C) during laser scanning operations isproduced by light rays of a particular polarization state (e.g. Spolarization state) associated with a diffracted laser beam beingscanned across a light reflective surface (e.g. the bars and spaces of abar code symbol) and scattering thereof, whereupon the polarizationstate distribution of the scattered light rays is typically altered whenthe scanned surface exhibits diffuse reflective characteristics.Thereafter, a portion of the scattered light rays are reflected alongthe same outgoing light ray paths toward the holographic facet whichproduced the scanned laser beam. These reflected light rays arecollected by the scanning facet and ultimately focused onto thephotodetector of the associated light detection subsystem by itsparabolic light reflecting mirror disposed beneath the scanning disc.The function of each photodetector is to detect variations in theamplitude (i.e. intensity) of optical scan data signal D₀, and producein response thereto an electrical analog scan data signal D₁ whichcorresponds to such intensity variations. When a photodetector withsuitable light sensitivity characteristics is used, the amplitudevariations of electrical analog scan data signal D₁ will linearlycorrespond to light reflection characteristics of the scanned surface(e.g. the scanned bar code symbol). The function of the analog signalprocessing circuitry is to band-pass filter and preamplify theelectrical analog scan data signal D₁, in order to improve the SNR ofthe output signal.

In the illustrative embodiment, each digital scan data signal processingboard 18A (18B and 18C) is constructed the same. On each of these signalprocessing boards, the following devices are realized. Ananalog-to-digital (A/D) conversion circuit 38A (38B, 38C) is realized asa first application specific integrated circuit (ASIC) chip. Aprogrammable digitizing circuit 39A (39B, 39C) is realized as a secondASIC chip. Also, a programmed decode computer 40A (40B, 40C) is realizedas a microprocessor and associated program and data storage memory andsystem buses, for carrying out symbol decoding operations. In theillustrative embodiment, the ASIC chips, the microprocessor, itsassociated memory and systems buses are all mounted on a single printedcircuit (PC) board, using suitable electrical connectors, in a mannerwell known in the art.

The function of the A/D conversion circuit is to perform a simplethresholding function in order to convert the electrical analog scandata signal D₁ into a corresponding digital scan data signal D₂ havingfirst and second (i.e. binary) signal levels which correspond to thebars and spaces of the bar code symbol being scanned. In practice, thedigital scan data signal D₂ appears as a pulse-width modulated typesignal as the first and second signal levels thereof vary in proportionto the width of bars and spaces in the scanned bar code symbol.

The function of the programmable digitizing circuit is to convert thedigital scan data signal D2, associated with each scanned bar codesymbol, into a corresponding sequence of digital words (i.e. a sequenceof digital count values) D₃. Notably, in the digital word sequence D3,each digital word represents the time length associated with each firstor second signal level in the corresponding digital scan data signal D₂.Preferably, these digital count values are in a suitable digital formatfor use in carrying out various symbol decoding operations which, likethe scanning pattern and volume of the present invention, will bedetermined primarily by the particular scanning application at hand.Reference is made to U.S. Pat. No. 5,343,027 to Knowles, incorporatedherein by reference, as it provides technical details regarding thedesign and construction of microelectronic digitizing circuits suitablefor use in the holographic laser scanner of the present invention.

In bar code symbol scanning applications, the function of the programmeddecode computer is to receive each digital word sequence D₃ producedfrom the digitizing circuit, and subject it to one or more bar codesymbol decoding algorithms in order to determine which bar code symbolis indicated (i.e. represented) by the digital word sequence D₃,originally derived from corresponding scan data signal D₁ detected bythe photodetector associated with the decode computer. In more generalscanning applications, the function of the programmed decode computer isto receive each digital word sequence D₃ produced from the digitizingcircuit, and subject it to one or more pattern recognition algorithms(e.g. character recognition algorithms) in order to determine whichpattern is indicated by the digital word sequence D₃. In bar code symbolreading applications, in which scanned code symbols can be any one of anumber of symbologies, a bar code symbol decoding algorithm withauto-discrimination capabilities can be used in a manner known in theart.

As shown in FIGS. 4A through 4C, the central processing board 21comprises a number of components mounted on a small PC board, namely: aprogrammed microprocessor 42 with a system bus and associated programand data storage memory, for controlling the system operation of theholographic laser scanner and performing other auxiliary functions;first, second, third and forth serial data channels 43, 44, 45 and 46,for receiving serial data input from the programmable decode computers40A (40B and 40C) and RF receiver/base unit 47; an input/output (I/O)interface circuit 48 for interfacing with and transmitting symbolcharacter data and other information to host computer system 24 (e.g.central computer, cash register, etc.); and a user-interface circuit 49for providing drive signals to an audio-transducer 50 and LED-basedvisual indicators 51 used to signal successful symbol reading operationsto users and the like. In the illustrative embodiment, each serial datachannel is be realized as an RS232 port, although it is understood thatother structures may be used to realize the function performed thereby.The programmed control computer 42 also produces motor control signals,and laser control signals during system operation. These control signalsare received as input by a power supply circuit 52 realized on the powersupply PC board 22, identified hereinabove. Other input signals to thepower supply circuit 52 include a 120 Volt, 60 Hz line voltage signalfrom a standard power distribution circuit. On the basis of the receivedinput signals, the power supply circuit produces as output, (1) lasersource enable signals to drive VLDs 53A, 53B and 53C, respectively, and(2) motor enable signals in order to drive the scanning disc motor 11.

In the illustrative embodiment, RF base unit 47 is realized on a verysmall PC board 54 mounted on the base plate 5 within the scannerhousing. Preferably, RF base unit 47 is constructed according to theteachings of copending U.S. application Ser. No. 08/292,237 filed Aug.17, 1995, also incorporated herein by reference. The function of thebase unit 47 is to receive data-packet modulated carrier signalstransmitted from a remotely situated bar code symbol reader, datacollection unit, or other device capable of transmitting data packetmodulated carrier signals of the type described in said application Ser.No. 08/292,237, supra.

In some holographic scanning applications, where omni-directionalscanning cannot be ensured at all regions within a prespecified scanningvolume, it may be useful to use scan data produced either (i) from thesame laser scanning plane reproduced many times over a very short timeduration while the code symbol is being scanned therethrough, or (ii)from several different scanning planes spatially contiguous within aprespecified portion of the scanning volume. In the first instance, ifthe bar code symbol is moved through a partial region of the scanningvolume, a number of partial scan data signal fragments associated withthe moved bar code symbol can be acquired by a particular scanning plane(e.g. P(i=1,j=3) being cyclically generated over an ultra-short periodof time (e.g. 1-3 milliseconds), thereby providing sufficient scan datato read the bar code symbol. In the second instance, if the bar codesymbol is within the scanning volume, a number of partial scan datasignal fragments associated with the bar code symbol can be acquired byseveral different scanning planes being simultaneously generated by thethree laser scanning stations of the system hereof, thereby providingsufficient scan data to read the bar code symbol, that is, provided suchscan data can be identified and collectively gathered at a particulardecode processor for symbol decoding operations.

In order to allow the holographic scanner of the present invention touse symbol decoding algorithms that operate upon partial scan datasignal fragments, as described above, the 0-th order signal detector andits associated processing circuitry are used to produce a periodicsignal X(t), as discussed briefly above. As the periodic signal X(t) isgenerated by the 0-th order of the incident laser beam passing throughthe outer radial portion of each holographic facet on the rotatingscanning disc, this signal will include a pulse at the occurrence ofeach holographic facet interface. However, in order to uniquely identifya particular facet for reference purposes, a “gap” of prespecified widthd_(gap), as shown in FIG. 3, is formed between two prespecified facets(i.e. i=2 and 16) at the radial distance through which the incidentlaser beam passes. Thus, in addition to the periodic inter-facet pulses,the periodic signal X(t) also includes a “synchronizing pulse” producedby the prespecified “gap” which is detectable every T=2π/ω [seconds],where ω is the constant angular velocity of the holographic scanningdisc maintained by the scanning disc motor and associated driver controlcircuitry. Thus, while the function of the 0-th order light detector isto detect the 0-th diffractive order of the incident laser beam, thefunction of its associated signal processing circuitry is to (1) detectthe periodic occurrence of the “synchronizing pulse” in the periodicsignal X(t) and (2) simultaneously generate a periodic synchronizingsignal S(t) containing only the periodic synchronizing pulse stream. Theconstruction of such pulse detection and signal generation circuitry iswell known within the ordinary skill of those in the art.

As each synchronizing pulse in the synchronizing signal S(t) issynchronous with the “reference” holographic facet on the scanning disc,the decode processor (i.e. computer) (40A, 40B, 40C) provided with thisperiodic signal can readily “link up” or relate, on a real-time basis,(1) each analog scan data signal D₁ it receives with (2) the particularholographic facet on the scanning disc that generated the analog scandata signal. To perform such signal-to-facet relating operations, thedecode computer is provided with information regarding the order inwhich the holographic facets are arranged on the scanning disc. Suchfacet order information can be represented as a sequence of facetnumbers (e.g. i=1, 16, 2, 15, 9, 12, 6, 11, 7, 10, 5, 8, 3, 13, 4,14, 1) stored within the associated memory of each decode processor. Byproducing both a scan data signal and a synchronizing signal S(t) asdescribed above, the holographic scanner of the present invention canreadily carry out a diverse repertoire of symbol decoding processeswhich use partial scan data signal fragments during the symbol readingprocess. The advantages of this feature of the system will becomeapparent hereinafter.

In code symbol reading applications where partial scan data signalfragments are used to decode scanned code symbols, the synchronizingsignal S(t) described above can be used to identify a set of digitalword sequences D₃, (i.e. {D_(S)}), associated with a set oftime-sequentially generated laser scanning beams produced by aparticular holographic facet on the scanning disc. In such applications,each set of digital word sequences can be used to decode a partiallyscanned code symbol and produce symbol character data representative ofthe scanned code symbol. In code symbol reading applications wherecomplete scan data signals are used to decode scanned code symbols, thesynchronizing signal S(t) described above need not be used, as thedigital word sequence D₃ corresponding to the completely scanned barcode symbol is sufficient to carry out symbol decoding operations usingconventional symbol decoding algorithms known in the art.

Description of the 3-D Laser Scanning Pattern of the IllustrativeEmbodiment of the Present Invention

Referring to FIG. 5, the laser scanning pattern generated by theholographic scanner hereof is illustrated in greater detail. Forillustrative purposes, the laser scanlines that are projected onto eachof the four focal planes of the scanning volume, are shown as blacklines labelled with their respective scanline (i.e. scanning plane)designation, P(i,j). Each such scanline has a scanline length which isdefined, for the most part, by the geometry of the scanning volumeV_(scanning), the boundaries of which are indicated by dotted lines, asshown. While the laser scanning pattern of the illustrative embodimenthas forty-eight scanning planes in total, only three scanning planes(i.e. scanlines) are simultaneously generated at any instant in time.However, within a single revolution of the holographic scanning disc,all forty-eight scanning planes are generated. The order in which eachscanning plane is produced during a single revolution of the scanningdisc is described by the schematic representation shown in FIG. 5A. Asindicated in this figure, the laser source and holographic facet used togenerate each scanning plane are indicated by its holographic facetnumber i and laser source number j.

It is appropriate at this juncture to now describe the cross-sectionalcharacteristics of the laser scanning pattern of the present invention,and the advantages provided thereby in omni-directional scanningapplications.

While the laser beam production module of the present invention providesa novel way to produce a circularized laser beam free of astigmatism dueto intrinsic properties of visible laser diodes (VLD), the laserscanning planes P(i,j) generated by the rotating holographic scanningdisc diffracting an astigmatism-free laser beam are not completely freeof astigmatism. By virtue of the fact that an incident collimated laserbeam is scanned through a light diffractive element at an angle ofincidence A_(i) other than zero degrees, results in astigmatism withinthe scanning volume. This form of astigmatism, referred to as “beam-scanastigmatism”, manifests itself at the end of each scanline and at theextreme portions of the depth of field for each set of scanlines.

While not necessarily apparent, there are several reasons why a zerodegree angle of incidence (i.e. A_(i)=0) cannot be used to eliminateastigmatism in the holographic scanner of the present invention. Thefirst reason is that this approach would greatly reduce the scan anglemultiplication factor M for each scanning facet, thus making itimpossible to achieve the scan pattern of the illustrative embodiment.Secondly, this approach would reduce the Total Light CollectionEfficiency of the facets, as the angles of diffraction B_(i) would haveto be lower to realize the spatially corresponding scanline. Thirdly,this approach would necessarily result in a holographic scanning discwhich would be extremely difficult to manufacture.

As shown in FIG. 6A, adjacent scanning planes overlap between focalregions within the scanning volume. Each scanning plane is produced aseach holographic facet is rotated through a circularized laser beamdirected incident thereto at about A_(i)=47° for all values of i. Whileeach scanning plane is often visualized as a continuous sheet of light,in actuality it is made up of a single laser beam whose movement isprogressively advanced while its cross-sectional dimensions are changedas the laser beam is diffracted through its scanline path in space.

Using the ZEMAX optical program from Focus Software, Inc. of Tucson,Ariz., the spot-diagrams of FIGS. 6B and 6C can be generated in order toanalyze the astigmatic characteristics of the scanned laser beamscomprising the scanning pattern of the present invention. As shown inFIGS. 6B and 6C, the spot size (i.e. cross-sectional) dimensions andorientation of a particular scanned laser beam are represented at itsfocal plane for five different distances along one half of the scanningplane, as well as for two planes above its focal plane and for twoplanes below its focal plane. In reality, the spacing of these scanningplanes from the focal plane are −120 mm, −60 mm, 60 mm, 120 mm,respectively. The five different spot-size distances represented alongthe scanning plane correspond to five different angular rotations of thescanning disc about its axis of rotation. Notably, spot-size diagramsshown in FIG. 6B are for a scanned laser beam having its focal planelocated farther out from the scanning window, whereas the spot-sizediagrams shown in FIG. 6C are for a scanned laser beam having its focalplane adjacent to the focal plane of FIG. 6B, and closer to the scanningwindow. The far right side of the spot-size diagram shown in FIGS. 6Aand 6B represent the middle of the neighboring scanning planes. Themiddle set of spot-size diagrams represent the cross-sectional diameterand orientation of the laser beam at its focal plane within the scanningvolume. The upper set of spot-size diagrams represent thecross-sectional diameter and orientation of the laser beam above itsfocal plane within the scanning volume. The lower set of spot-sizediagrams represent the cross-sectional diameter and orientation of thelaser beam below its focal plane within the scanning volume.

In each of the spot-size diagrams shown in FIGS. 6B and 6C, the beamorientations are governed by the astigmatism introduced as the incidentlaser beam is diffracted by its corresponding holographic facet movingabout the disk axis of rotation. At each focal plane in the scanningvolume, a particular laser beam is focused thereat with astigmaticcharacteristics that are opposite those of the neighboring laser beamwhich spatially overlaps the particular laser beam. As illustrated inFIGS. 6A and 6B, the direction of beam orientation, measured from themiddle of the scan line, at the focal plane, rotates in a directionopposite the direction that the neighboring overlapping laser beamrotates. Consequently, in the region of overlapping laser beams betweeneach adjacent pair of focal planes within the scanning volume, thecomplementary beam cross-sectional characteristics cooperate to providean omni-directional scanning field over the extent of the spatiallyoverlapping scanning planes. Thus, when a bar code to be scanned isoriented in a manner which makes it difficult to read the symbol due tothe tilt of the astigmatic spot in the near portion of the two adjacentfocal regions, the tilt of the astigmatic spot in the adjacent far fieldregion is in the opposite direction, making it easier to read the samecode symbol. Collectively, the overlapping scanning planes betweenadjacent focal regions within the scanning volume provides robustomni-directional code symbol scanning performance.

Designing a Holographic Laser Scanning System According to the Method ofthe Present Invention

In FIG. 7, the four primary steps involved in designing a holographiclaser scanner according to the present invention are shown.

As indicated at Block A in FIG. 7, the first step of the design methodinvolves geometrically specifying the following entities: (i) thestructure of the three-dimensional scanning pattern and scanning volumeto be realized; (ii) the performance parameters of the scanner to bedesigned; and (iii) the volumetric dimensions of the scanner housingfrom which the scanning pattern is to be generated. Typically, each ofthese entities will be specified by end user requirements which includefactors such as: the scanning application and environment at hand; barcode symbol resolution; reflection characteristics of bar code symbolsubstrate; speed of objects being identified; and the throughput of thescanning environment.

Thus, as part of this specification step, the number and location ofeach scanning plane (i.e. focal plane), and its focal distance f_(i)within the specified scanning volume must be specified in geometricalterms, that is, using coordinate geometry etc. In general, this stepinvolves providing a geometrical specification of the 3-D laser scanningpattern, as shown in FIGS. 5, 6A, 6B and 6C, for example. In short, thisprocedure necessitates specifying a coordinate system (e.g. Cartesiancoordinate system), and then specifying the location of each scan line(i.e. scanning plane) within the scanning volume and its focal distancef_(i) from the i-th holographic scanning facet. Naturally, theresolution of the bar code symbols to be read will determine the largestcross sectional dimension that each scan line can be in order to resolvethe bar code symbol. Thus, it will be necessary to provide a properspecification of the maximum cross-sectional diameter of the scannedlaser beams within the operative scanning range of the specifiedscanning volume.

As shown in FIG. 3, the scanning pattern of the illustrative embodimenthas four specified focal planes, indexed as k=1,2,3,4. Each of the scanlines within each of the focal planes is specified in terms of itsgeometrical coordinates. For example, four focal planes are used in theillustrative embodiment to satisfy a 40 inch depth of field requirementfor the exemplary application at hand. While this may appearconservative at first, it has been found that this four focal planedesign offers an important advantage over other system designs in thatit provides a vertical “sweet spot” in the central portion of the 3-Dscanning volume. In the illustrative embodiment, each of these fourfocal planes are parallel to the scanning window of the scanner, andeach of the four scan patterns in the four focal planes are centeredover the rotational axis of the rotating holographic disk. Also, thelines at each focal plane are spaced equally apart from each other. Thebasic four line scan pattern selected in the illustrative embodimentprovides good coverage of the scan region at each focal plane. From thecustomer requirements, the minimum and maximum focal distances andlengths of each scanline S_(L) in the scanning volume V_(scanning) canbe established (i.e. determined) in order to completely cover each ofthe scanning regions in the scanning volume.

As indicated at Block B of FIG. 7, the next step of the design methodinvolves selecting a basic architecture for the laser scanning platformupon which the designed scanning pattern will be produced. In theillustrative embodiment shown in FIGS. 1 through 4, the laser scanningapparatus selected as a suitable laser scanning platform for theenvisioned 3-D scanning pattern, comprises three symmetrical laserscanning stations constructed about the holographic scanning disc of thepresent invention, each of the laser scanning stations has a laser beamproduction module and light collecting and detecting subsystem. Thethree laser scanning station architecture adopted in the illustrativeembodiment, provides the best method for generating the bar-X scanpattern of the exemplary scan pattern. The symmetry of the scanningpattern dictates that all three laser scanning channels should be thesame, allowing that the design for any one channel be the same as thatfor the other three channels. For the sake of convenience, the scanpattern created at each of the focal planes should be centered over therotational axis of the holographic scanning disk, although it isunderstood that this is not a necessary condition. As will be shownhereinafter, the design method of the present invention allows one toeasily change system parameters so that the axially centered scanningpattern can be changed to a non-centered location, or the scan patterncan be configured in a non-symmetrical manner, away from the axis ofrotation of the holographic scanning disk.

Having specified the 3-D scanning pattern and platform architecture fora given application, the next step in the scanner design methodindicated in FIG. 7 hereof involves using the scanning pattern andvolume specifications and scanner housing specifications to design aparticular scanning platform comprising a holographic scanning disc ofthe present invention and an array of beam folding mirrors configured insuch a manner so the resultant system produces the specified scanningpattern. Preferred disk design methods will be described in great detailbelow with reference to FIGS. 8A through 12C. Also, a preferred methodof constructing the designed scanning disk will be described thereafterwith reference to FIG. 13.

As indicated at Block D in FIG. 7, the next step of the method involvesdesigning a laser beam production module using the holographic scanningdisk specifications acquired at Block B. Notably, the scanning diskspecifications required during this step of the design method includethe angle of incidence A_(i) for each facet, the angle of diffractionB_(i) thereof, and the central wavelength λ_(i) of the laser beamproduced from the VLD. As will be described in great detail hereinafter,the function of the laser production module is to produce an incidentlaser beam that has a circularized (or aspect-ratio controlled) beamcross-section, is free from the effects of astigmatism along itsoperative scanning range, and, which, in conjunction with the laserscanning disk, minimizes dispersion of spectral components thereof asthe laser beam is diffractively transmitted through the facets along therotating scanning disk. In the illustrative embodiments, two differenttechniques are employed in order to realize the above describedfunctions utilizing ultra-compact structures. In the first illustrativeembodiment of the present invention shown in FIGS. 14 through 21D, aVLD, an aspherical lens, a beam expanding prism, a light diffractivegrating of fixed spatial frequency are used to construct the laser beamproduction module of the present invention. In the second illustrativeembodiment of the present invention shown in FIGS. 22 through 31D, anaspherical lens, and a multi-function light diffractive grating of fixedspatial frequency are used to construct the laser beam production modulehereof. In both embodiments, novel design techniques are employed which,for the first time, allow the use of conventional VLDs in a holographiccode symbol reading system without sacrificing high performancecharacteristics.

As indicated in Block E in FIG. 7, the last step of the design methodinvolves specifying and designing a light collecting and detectingsubsystem (hereinafter “light detection subsystem”) for use with thedesigned holographic laser scanner. As will be described in greaterdetail hereinafter with reference to FIGS. 32 through 43B, severaldifferent types of subsystems may be used to realize this systemcomponent in accordance with the principles of the present invention.

In the first preferred embodiment of the light collecting and detectingsubsystem, a parabolic mirror is disposed beneath the light collectingarea of the scanning disk and is designed to focus incoming collectedlight rays towards a photodetector disposed at the focal length of theparabolic mirror above the scanning disk. The focal characteristics ofthe parabolic mirror and its position relative to the scanning disc arechosen so that each focused light ray is transmitted through thescanning disk at an angle of incidence which minimizes the lightdiffraction efficiency thereof. In the second illustrative embodiment ofthe light collecting and detecting subsystem, a reflective-volume typeholographic diffraction grating of variable spatial frequency isdisposed beneath the light collecting area of the scanning disk and isdesigned to focus incoming collected light rays towards a photodetectordisposed at the focal length of the reflection-volume type holographicgrating above the scanning disk. The focal characteristics of theparabolic reflection-volume hologram and its position relative to thescanning disc are chosen such that each focused light ray is transmittedthrough the scanning disk at an angle of incidence which minimizes thelight diffraction efficiency thereof. The third illustrative embodimentof the light collecting and detecting subsystem includes a planarmirror, light focusing optics and a photodetector disposed beneath thelight collecting area of the scanning disk. Each of these embodimentswill be described in detail hereinafter with reference to FIGS. 32through 43B.

Referring to FIGS. 11A through 11C, the major steps involved inpracticing the “holographic scanner” design method hereof will now bedescribed in great detail. Notably, this term is used herein to describethe overall process used to design all of the subsystems of theholographic laser scanner including, but not limited to, the holographicscanning disc, the beam folding mirror array, the light collecting anddetecting subsystem, the laser beam production modules, as well as thescanner housing within which such subsystems are contained. Thus, theholographic scanner design method hereof comprises a collection ofsubsystem design methods and processes which interact with each other toprovide a composite method. In general, there are numerous embodimentsof the holographic scanner design method of the present invention.Factors which influence the design of the scanning disc and lightdetection subsystem include, for example, the polarization state of theincident laser beam used during scanning operations, as well as thepolarization state of the laser light rays collected, focused anddetected by the light collecting and detecting subsystem used duringlight collecting and detecting operations.

In the illustrative embodiments of the present invention, the scannerdesign methods hereof are carried out on a computer-aided design (CAD)workstation which can be realized using a computer system, such as theMacintosh 8500/120 computer system. In the illustrative embodiment, theCAD-workstation supports a 3-D geometrical database for storing andretrieving information representative of 3-D models of the holographicscanning apparatus and processes under design; as well as a relationaldatabase for storing and retrieving information representative ofgeometrical and analytical models holographic laser scanning apparatusand processes under design. In addition, the CAD workstation includes adiverse array of computer programs which, when executed, provide anumber of important design and analysis tools. Such design and analysistools include, but are not limited to: 3-D geometrical modelling tools(e.g. AUTOCAD geometrical modelling software, by AutoDesk, Inc. forcreating and modifying 3-D geometrical models of virtually every aspectof the holographic laser scanning apparatus and processes under design;robust mathematical modelling tools (e.g. MATHCAD 3.1 for Macintosh byMathSoft, Inc. of Cambridge, Mass.) for creating, modifying andanalyzing mathematical models of the holographic scanning apparatus andprocesses under design; and spreadsheet modelling tools (e.g. EXCEL byMicrosoft Corporation, or LOTUS by Lotus Development Corporation) forcreating, modifying and analyzing spreadsheet-type analytical models ofthe holographic scanning apparatus and processes under design. Forpurposes of simplicity of expression, the above-described CADworkstation and all of its tools shall be collectively referred to asthe “Holographic Scanner Design (HSD) workstation” of the presentinvention. Where necessary or otherwise appropriate, the functionalitiesand tools of the HSD workstation will be elaborated in greater detailhereinafter.

As indicated in Block A of FIG. 11A, the first step of the scannerdesign method involves the scanner designer creating within thegeometrical database of the HSD workstation hereof, a geometrical modelof the holographic laser scanner described above. Preferably, a 3-Dgeometrical model of the holographic laser scanner, including thescanning disc, is created, although a 2-D geometrical model will sufficein many applications where the symmetry of the scanning apparatus allowssuch simplification. A schematic diagram of the geometrical model of theholographic scanning disc under design is set forth in FIG. 9. Usingthis geometrical model of the scanning disc, the scanner designer thenproceeds to index each i-th holographic facet on the scanning disc, aswell as each j-th laser beam production module within the holographicscanning system. In the illustrative embodiment, this two-fold indexingstep is carried out by assigning a unique number to each facet on theholographic scanning disc under design, and a unique number to eachlaser beam production module employed in the holographic laser scanningsystem of the present invention. The assigned facet and laser beamproduction module indices can then be used to identify which facets andlaser beams are being referred to during the design and constructionprocesses.

As indicated at Block B of FIG. 11A, the scanner designer then begins tocreate within the geometrical database of the HSD workstation, ageometrical model of the 3-D laser scanning pattern production processrealized upon the multi-station laser scanning platform of the presentinvention. Owing to the symmetry of the laser scanning platform hereof,modelling of the complex laser scanning process of the present inventioncan be readily simplified by separately modelling the generation of each(i,j)th scanline within the 3-D laser scanning volume. Inasmuch as each(i,j)-th scanline is generated in substantially the same manner, exceptfor the fact that a different (i-th) facet and a particular (j-th) laserbeam are used to generate each scanline in the scanning volume, thesubstantially same geometrical optics model shown in FIG. 8A can be usedto represent the production of each (i,j)th scanline.

In general, the geometrical optics model used to represent each (i,j)thscanline generation process employs a geometrical specification of thefollowing structures: (1) the (i,j)-th scanline in physical relation tothe stationary laser beam production module, the corresponding facet onthe rotating holographic disc, the stationary beam folding mirror, andthe base and scanning window of the scanner housing; and (2) the raydiagram tracing the path of the incident j-th laser beam from the laserbeam production module, through the i-th facet, off the j-th beamfolding mirror, and focusing onto the focal plane along which the(i,j)-th scanline extends. In order to eliminate the need forconsidering the reflection of the rays at the surface of the foldingmirrors, and thus simplify the disk design process, a virtualholographic scanning disk 56 is defined relative to the real holographicscanning disk, as shown in FIGS. 8A and 8A1. This modelling techniqueallows the subsequent calculations to be made using the locations of theBeam Incident Point r₀ and the Inner Radii of the facets r_(i) in thevirtual disk.

As part of the geometrical modelling process called for in Block B ofFIG. 11A, numerous geometrical parameters and analytical equationsdefining relations therebetween need to be carefully defined by thescanner designer for use during the subsequent stages of the designprocess. In FIGS. 8B1 and 8B2, the parameters used to construct thegeometrical model are defined. In FIGS. 8C1 and 8C2, the set ofmathematical expressions used to establish important relations amongcertain of the parameters in the model are listed in a specifiednumerical order for future reference herein. The set of mathematicalexpressions set forth in FIGS. 8C1 and 8C2 provide an analytical modelfor the scanline production process of the present invention.

As indicated in FIGS. 8B1 and 8B2, the parameters used to construct thegeometrical model of the (i,j)th scanline production process include:

(1) the radius to the Beam-Incident-Point on the holographic scanningdisc, assigned the symbolic notated “r₀”;

(2) Scanline Separation between adjacent scanlines at the focal plane ofthe (i,j)-th scanline, assigned the symbolic notated “S_(SL)”;

(3) the Scanline Length (measured into the paper) for the (i,j)thscanline, assigned the symbolic notation “L_(SL)”;

(4) the Distance measured from the scanning disc to the focal plane ofthe (i,j)th scanline, assigned the symbolic notation “a_(i)”;

(5) the Distance from radius to the Beam-Incident Point r₀ to beamfolding mirror, assigned the symbolic notated “L”;

(6) the Tilt Angle of the j-th beam folding mirror associated with thegeneration of the (i,j)-th scanline, assigned the symbolic notation“φ_(j)”;

(7) the Tilt Angle of the virtual scanning disc, assigned the symbolicnotation “2φ”;

(8) the Lateral Shift of the Beam Incident Point on the virtual scanningdisc, assigned the symbolic notation “Δx”;

(9) the Vertical Shift of the Beam Incident Point on the virtualscanning disc, assigned the symbolic notation “Δy”;

(10) the Distance from the rotation axis to the Beam Incident Point onthe virtual scanning disc, assigned the symbolic notation r_(o)+Δx;

(11) the Distance from the Beam-Incident-Point on the virtual scanningdisc to the focal plane within which the (i,j)th scanline resides,assigned the symbolic notation f_(i);

(12) the Diameter of the cross-section of the laser beam at the scanningdisc, produced from the j-th laser beam scanning station, assigned thesymbolic notation “d_(beam)”;

(13) the Angular Gap between adjacent holographic scanning facets,assigned the symbolic notation “d_(gap)”;

(14) the Outer Radius of the available light collection region on theholographic scanning disc, assigned the symbolic notation “r_(outer)”;

(15) the Inner Radius of the available light collection region on theholographic scanning facet, assigned the symbolic notation “r_(inner)”;

(16) one-half of the Depth of Field of the (i,j)th scanline, assignedthe symbolic notation “δ”;

(17) the Distance from the maximum read distance (f_(i)+δ=5″) to theInner Radius r_(i) of the scanning facet, assigned the symbolic notation“C”;

(18) the Outer Ray Angle measured relative to the normal to the i-thholographic facet, assigned the symbolic notation “α”;

(19) the Inner Ray Angle measured relative to the normal to the i-thholographic scanning facet, assigned the symbolic notation “γ”;

(20) the Light Collection Angle, measured from the focal point +δ of thei-th facet to the light collection area of the scanning facet, assignedthe symbolic notation “β”;

(21) the intersection of the beam folding mirror and line C, assignedthe symbolic notation “x” (x measured from rotational axis of disk);

(21) the intersection of the beam folding mirror and line C, assignedthe symbolic notation “y” (y measured from plane of disk);

(22) the Distance measured from the Inner Radius to the point of mirrorintersection, assigned the symbolic notation “D”;

(23) the Distance measured from the base of the scanner housing to thetop of the j-th beam folding mirror, assigned the symbolic notation “h”;

(24) the Distance measured from the scanning disk to the base of theholographic scanner, assigned the symbolic notation “d”;

(25) the Focal Length of the i-th holographic scanning facet from thescanning facet to the corresponding focal plane within the scanningvolume, assigned the symbolic notation “f_(i)”;

(26) Incident Beam Angle measured with respect to the i-th holographicfacet surface, assigned the symbolic notation “A_(i)”;

(27) Diffracted Beam Angle measured with respect to the i-th holographicfacet surface, assigned the symbolic notation “B_(i)”;

(28) the Angle of the j-th laser beam measured from the vertical,assigned the symbolic notation “−α”;

(29) the Scan Angle of the diffracted laser beam produced by i-th facet,assigned the symbolic notation “θ_(si)”;

(30) the Scan Multiplication Factor for the i-th holographic facet,assigned the symbolic notation “M_(i)”;

(31) the Facet Rotation Angle for the i-th holographic facet, assignedthe symbolic notation “θ_(roti)”;

(32) Adjusted Facet Rotation Angle accounting for deadtime, assigned thesymbolic notation “θ′_(roti)”;

(33) the Light Collection Efficiency factor for the i-th holographicfacet, normalized relative to the 16th facet, assigned the symbolicnotation “ζ_(i)”

(34) the Total Light Collection Area for the i-th holographic facet,assigned the symbolic notation “Area Total_(i)”;

(35) the Beam Speed at the Center of the (i,j)th Scanline, assigned thesymbolic notation “v_(center)”;

(36) the Angle of Skew of the diffracted laser beam at the center of thei-th holographic facet, assigned the symbolic notation “θ_(skew)”;

(37) the Maximum Beam Speed of all laser beams produced by theholographic scanning disc, assigned the symbolic notation “v_(max)”;

(38) the Minimum Beam Speed of all laser beams produced by theholographic scanning disc, assigned the symbolic notation “v_(min)”;

(39) the ratio of the Maximum Beam Speed to the Minimum beam speed,assigned the symbolic notation “v_(max)/v_(min)”; and

(40) the deviation of the light rays reflected off the parabolic lightreflecting mirror beneath the scanning disc, from the Bragg angle forthe facet assigned the symbolic notation “δ_(e)”.

Notably, certain of the above-defined parameters are assignedinitialized (i.e. assumed) values, whereas other parameters are computedusing the mathematical expressions set forth in FIGS. 8C1 and C2.Exactly which parameters are initialized, and which are computed, and inwhat order, will be explained hereinafter.

As indicated at Block C in FIG. 11A, the next step of the scanner designprocess involves using the geometrical parameters and mathematicalexpressions of FIGS. 8B1 through 8C2, and the “spreadsheet” modellingtool of the HSD workstation in order to create an analytically-basedScanline Production Model which describes the physical production ofeach (i,j)th scanline within the 3-D scanning volume of the presentinvention. As mentioned above, suitable spreadsheet-computer programsfor carrying out this stage of the disc design process include, forexample, EXCEL® from Microsoft, Inc., and LOTUS® from the LotusDevelopment Corporation. The function of the spreadsheetmodelling/analysis tool is to provide a network-type information storagestructure, within which the mathematical expressions of thespreadsheet-based Scanline Production Model can be embodied in a mannerwell known in the spreadsheet computing art. With functional linksestablished among the information storage nodes within the underlyinginformation storage network of the spreadsheet computer program, thescanner designer is thereby permitted to modify one or more parametersof the analytical model and analyze how other parameters within themodel change, permitting “what if” analysis with respect to the variousparameters comprising the analytical Scanline Production Model. Notably,the display format for the spreadsheet tool will vary from embodiment toembodiment, and in itself, is not an important aspect of the presentinvention.

As indicated at Block D of FIG. 11A, the next step of the disc designprocess involves the scanner designer specifying assumed (i.e. initial)values for a number of parameters in the spreadsheet-type analyticalmodel of each (i,j)th scan line production process. In the illustrativeembodiment, these assumed parameters include: the radius toBeam-Incident-Point on the holographic scanning disc r₀, which by designis the same for each (i,j)-th scanline (mainly determined by the size ofthe disk); Scanline Separation S_(SL) of adjacent scanlines at the focalplane of the (i,j)-th scanline, and “ScanLine Length” for the (i,j)-thscanline L_(SL) (both established by the user application requirements);Distance from the Beam-Incident-Point to the Beam Folding Mirror, L(usually chosen to be as small as possible to minimize scanner volume);Tilt Angle of Beam Folding Mirror associated with the generation of the(i,j)-th scanline, φ_(j); the distance from the scanning disc to thefocal plane of the (i,j)-th scanline, f_(i); the cross-sectionaldiameter of the laser beam d_(beam) produced from the j-th laser beamscanning station (established by spot size requirements at the focalplanes); the Angular Gap between adjacent holographic scanning facets,d_(gap), and the width of the Home-Pulse Gap d_(gap)max; the OuterRadius of the light collection region on the holographic scanning disc,r_(outer); one-half (½) of the Depth of Field of the (i,j)th scanline,δ; the Distance from the holographic scanning disc to the Base of theholographic laser scanner, d; and the Deviation Angle δ_(e), from theBragg angle. Notably, the assumed values for these parameters areselected using both heuristics and experience associated with eachparticular parameter. Typically, such heuristics are obtained fromdesign criteria and scanner application requirements of the end user.Such heuristics will be briefly discussed below.

In general, the diameter of the holographic sscanning disk can beinitially selected on the basis of estimates of the required Lambertianlight collection efficiency of the holographic scanning facets, and theuseable optical power producible from commercially available VLDs. Inthe illustrative embodiment, a 220 mm diameter was selected for theholographic scanning disk. This assumed figure was a compromise betweenmaximizing the diameter of the scanning disk in order to maximize theLambertian light collection efficiency, and minimizing the diameter ofthe scanning disk to provide a more compact scanner housing design whileminimizing mechanical problems. Then initial values were selected forthe Angular Gap d_(gap) between adjacent holographic facets, and thewidth of the Home-Pulse Gap d_(gap)max.

Once assumed values have been established for the above-describedparameters, the balance of the “initializable” parameters in thespreadsheet-based Scanline Production Model can be determined usingfundamental geometric and/or trigonometric equations. For example, thegeometrical parameters Δx, Δy, indirectly specifying the location of thevirtual image of the scanning disk created by the folding mirror, can beestablished (i.e. initialized) by applying the Laws of Reflection. Thelocation of the Scanline Center Points (x,y,z) can be determined fromthe initialized Scanline Spacings S_(SL), the assumed Focal Distancesfor the scanning facets f_(i), and the symmetry of the axially centeredscan pattern of the illustrative embodiment. In order to allow symbolreading at the limit of the depth of field for each scanning plane, eachfocal distance f_(i) to the (i,j)th specified scanline should beslightly extended (e.g. by 5 inches).

Having created at Block D of FIG. 11D, a spreadsheet model for the(i,j)th scanline production process, the scanner designer then uses thespreadsheet tool of the HSD workstation to automatically compute thevalue of parameters in the Scanline Production Model using dependentparameters which are known by either assumption (i.e. initialization) ornumerical evaluation. While the order in which particular parameters ofthe analytical model are numerically evaluated (due to parametricdependency) is generally transparent to the operator of the spreadsheettool, the scanner designer of the spreadsheet-based Scanline ProductionModel must know the relational dependency among the various parametersin the analytical structures thereof so that the information nodes andfields underlying the spreadsheet model can be properly structured. Thusfor purposes of clarity and completeness, the computational stepscarried out within the spreadsheet-based Scanline Production Model ofthe present invention during the scanner design process will bedescribed in detail below. It is understood, however, that in practice,many of these steps will be transparent to the scanner designer inasmuchas he or she will need to provide particular inputs into thespreadsheet-based Scanline Production Model, and the Model willautomatically produce for display, parameters of relevance to thescanner design process.

Having assumed initial values for the above-described parameters atBlock D in FIG. 11A, the next step of the design process hereof,indicated at Block E thereof, is to use Expression No. 17 in FIG. 8C2,the mathematical expressions dependent therefrom (Nos. 16,15,14,13,12,and 1) and the assumed dependent parameters within the ScanlineProduction Model to numerically evaluate the Scan Angle θ_(si) requiredto produce the specified ScanLineLength L_(SL) associated with each i-thholographic facet. As reflected by this set of functionally dependentexpressions, the Scan Angle θ_(si) required to produce the specifiedScanLine Length L_(SL) is determined solely by the selection of assumedvalues for the parameters indicated in Expression Nos.17,16,15,14,13,12, and 1.

A few observations at this point will be helpful. First, for a givenScan Angle θ_(si), it is possible to adjust the Scanline Length L_(LS)at the focal plane specified by focal length f_(i) by simply increasingor decreasing the Scanline Multiplication factor M_(i), which isdependent upon the Angle of Incidence A_(i) and the Angle of DiffractionB_(i). Secondly, the sum of the Adjusted Facet Rotation Angles,θ′_(rot), for all of the facets (including the sweep angle associatedwith dead time, θ_(dead)=d_(beam)/r_(o)+d_(gap)/r_(o)) should equalapproximately 358.5 degrees in an optimum design. This allows for 1.5degrees extra for the large interfaced gap used for the home pulse. Ifthis total is more than 358.5 degrees, the proposed design will beinadequate. If the total is less than 358.5 degrees, the beam speedswill be unnecessarily high.

As indicated at Block F in FIG. 11A, the next step of the scanner designstep is to numerically evaluate, for each i-th scanning facet, theDiffraction (Outgoing) Beam Angle B_(i) associated with the i-thscanning facet. This computation is carried out using Expression No. 13in FIG. 8C2 and previously assumed and evaluated parameters specified bythis mathematical expression. Completion of this step produces aDiffracted (Outgoing Beam) Angle B_(i) for each of the 16 facets for thescanning disk under design. Notably, both Angles of Incidence andDiffraction A_(i) and B_(i) must provide the required ScanLine LengthL_(SL), with no excess. There is a subtle relationship between theseangles and the speed of the laser beam being moved along the scanlineduring scanner operation. In particular, if the angle of incidence A_(i)is increased below a particular value, then the scan pattern may not beadequate for the application at hand. On the other hand, if the angle ofincidence A_(i) were decreased, the scan pattern may be longer thannecessary, resulting in higher than necessary scan beam velocities. Thecorrect value of A_(i) will minimize the beam velocity at the focalplanes of each of the scan patterns, which in turn minimizes therequired electronic-bandwidth for the signal circuitry connected to thephotodetectors.

After completing this computational step, the scanner designer uses aMATHCAD-based program running on the HSD workstation to numericallyevaluate at Block G, for each i-th holographic facet, the relative LightDiffraction Efficiency Factor thereof H_(i) to light of a particularpolarization state. In order to compute these parameters {H_(i)}, thespreadsheet-based Scanline Production Model employs a computersub-program to perform a light diffraction efficiency analysis upon eachof the scanning facets under design, and computes therefrom, the totalout-and-back light diffraction efficiency of the i-th scanning facetrelative to the total out-and-back light diffraction efficiency of the16th scanning facet, to provide a normalized light diffractionefficiency measure for the i-th facet. This computational processinvolves theoretically deriving mathematical expressions representativeof the light diffraction efficiency of each scanning facet, that is,given the polarization state and light detection scheme employed in theparticular scanner embodiment at hand. The details of this analysis willbe explained below.

In FIG. 10A1, a geometrical optics model is provided for relative lightdiffraction efficiency (H_(i)) calculations in the case where theincident laser beam is produced from a VLD generating an S polarizedlight beam, and no polarizing filter is provided in front of thephotodetector of each scanning station. This figure drawing shows theoptical paths along which the laser beam is diffracted, reflected,diffracted, focused and transmitted without substantial diffractionduring the laser beam scanning and light collection process of thepresent invention. The transformation of polarization states during thisprocess is described in FIG. 10A. The mathematical expression used tocompute the light diffraction efficiency of each i-th scanning facet toS and P polarized light is derived from the geometrical optics modelshown in FIGS. 10A2 and 10A3 and the analytical model (i.e. tool) isdescribed in FIGS. 10B through 10E2.

In the preferred embodiment, the analytical model of FIGS. 10B through10E2 is realized using MATHCAD 3.1 mathematical modelling programavailable from MathSoft, Inc, of Cambridge, Mass. The mathematicalexpression derived for the total out-and-back diffraction efficiency foran S-polarized outgoing beam incident on the scanning disk (includingFresnel reflection losses and other internal losses of 10%) is notatedas T_(S)[Δn_(i)] and is set forth in Expression No. 13 in FIG. 10C2.Notably, in the geometrical optics model used to support the diffractionefficiency analysis, angle of incidence θ_(i) and angles of diffractionθ_(d) are defined differently from angle of incidence A_(i) and angle ofdiffraction B_(i) used in the Scanline Production Model described above.This fact is based solely on historical reasons, and is of littlesignificance. However, such angles are mathematically related angles.Angles A_(i) and B_(i) are complements of angles θ_(i) and θ_(d),respectively, and thus A_(i)=90°−θ_(i), and B_(i)=90°−θ_(d). As shown,this mathematical expression depends on the S polarization and Ppolarization diffraction efficiencies of the i-th holographic facets onthe disk which, in general, are functions of various parameters,including the incidence angle θ_(i) and the modulation index (i.e.modulation depth or fringe contrast) Δn_(i) of the holographic facet,assuming the thickness of the emulsion T is maintained constant acrossthe facet. However, by fixing (assuming a value for) each of thevariables in the expressions for these diffraction efficiencies, exceptΔn_(i), the expressions for these diffraction efficiencies can be madesimply a function of Δn_(i). In such circumstances, the lightdiffraction efficiency can be set by simply controlling the modulationindex Δn_(i) during facet construction in a holographic laboratory. In amanner well known in the art, the modulation index Δn_(i) can becontrolled by properly exposing and processing the dichromated gelatin(DCG) used to record the fringe structure of the scanning facet. Thenecessary exposure control can be achieved by controlling the power ofthe construction laser beam and/or time duration that the laser beam isincident on the gelatin during the holographic recording operation.

Expression No. 14 in FIG. 10C2 sets forth how to compute the relativelight diffraction efficiency factor H_(i) for each facet as a functionof the total out-and-back diffraction efficiencies T_(S)[Δn_(i)] foreach i-th and 16-th scanning facets. However, it will be appropriate tofirst describe techniques that can be used to derive mathematicalexpressions No. 11 and 12 in FIG. 10C2 for S and P polarizationdiffraction efficiencies in the holographic laser scanner system underdesign.

Foremost, it is important to be clear as to the referencing of the S andP polarization directions when deriving mathematical expressions for theS and P light diffraction efficiencies of holographic scanning facets.In accordance with convention, these polarization directions are definedwith respect to the plane of incidence, namely: the “S polarizationdirection” is defined to reside in the direction perpendicular to the“plane of incidence”; whereas the “P polarization direction” is definedto reside in the direction parallel to the plane of incidence. The“plane of incidence” is defined as that plane containing both the normalto the facet surface, at the point of incidence of the incident ray, andthe incident ray. Also, it is important to keep clear in mind that suchpolarization directions refer to the direction in which the ElectricField (or E-field) vector associated with the spherical wavefront of theincident laser beam acts on static electric charges duringelectromagnetic wave propagation.

To avoid confusion with the S&P terms introduced in a later sectionconcerning the astigmatic sources in a VLD, the terms “S wave-Component”and “P wave-component” will be introduced to define the abovepolarization directions of the incident laser beam. The term “Swave-component” is used to specify the component of the resultantspherical wavefront emanating from the laser beam production module andfalling incident upon the scanning disc, and having an E-field vectororiented in the S polarization direction. Similarly, the term “Spolarized wave-component” is used to specify the component of theresultant spherical wavefront emanating from the laser beam productionmodule and falling incident upon the scanning disc, and having anE-field vector oriented in the P-polarization direction. According tosuch definitions, both the S and P cylindrical wavefronts comprising theresulting spherical wavefront of the incident laser beam will contributeto the S wave-component, whereas both the S and P cylindrical wavefrontscomprising the resulting spherical wavefront of the incident laser beamwill contribute to the P wave-component.

The model used herein to describe the total out-and back diffractionefficiency of S and P wave-components of the incident laser beam duringscanning operations is based upon the theory of electromagnetic-wavecoupling within thick holographic structures, which was originallydescribed in the celebrated paper entitled “Coupled Wave Theory forThick Hologram Grating” by Herwig Kogelnik, supra. The two basicassumptions upon which this theory requires for application are: (1)that the thickness T of the emulsion in which the holographic fringestructures are formed is substantially greater than the wavelength ofthe incident wavefront; and (2) that the incident wavefront can beapproximated by a parallel wavefront. The first assumption holds truefor our volume-transmission type holograms, from which each holographicfacet on the scanning disc hereof is made. The second assumption alsoholds true for the case where the spherical wavefront incident the inputsurface of the hologram has a very large radius of curvature over theincident surface, which is true in the present invention.

In FIG. 10C1, a set of mathematical expressions are provided. Thesemathematical expressions are used to derive light diffraction efficiencyexpressions identified by Expressions 11, 12 and 13 in FIG. 10C2.Expression No. 1 through 3 in FIG. 10C1 relate internal angles toexternal angles through Snell's Law. Expressions 4 and 5 describeattributes of the slanted fringe structure of the holographic lightdiffraction facet sandwiched between the glass support plates of thescanning disc. These expressions have been derived by applying Snell'sLaw at the interfacial surfaces of the scanning disc, and using the wellknown Grating Equation to derive the variable spatial frequency fringestructure of the scanning facet. Expressions No. 6 through 10 in FIG.10C1 relate the coupling of incident and diffracted wave to the internalangle α and fringe slant angle φ associated with the scanning facet, andare derived from the fundamental work of Kogelnik, supra. Notably, theobliquity factors set forth in Expressions No. 6 and 7 are expressed asa function of the internal angle α and fringe slant angle φ for aparticular scanning facet, and determine how well optical input power isdiffracted in the various diffraction orders. While the total out andback light diffraction efficiencies defined by Expressions No. 11 and 12are functions of modulation depth, it is understood that such lightdiffraction efficiency expressions can be derived as a function of angleof incidence, as required in Bragg Sensitivity Analysis, by fixing themodulation index, Δn (i.e. Δn=n, in graphical plots), and allowing δ, inExpression No. 9 to vary.

Notably, Expressions 11 and 12 in FIG. 10C2 include three terms. Thefirst term in both of these mathematical expressions is a function offactors N(Δn) and S(Δn) defined by Expression Nos. 8 and 10 in FIG.10C1, and relates to the transmission of light by way of the process oflight diffraction, as explained in terms of the Coupled Wave Theorydescribed by Kogelnik, supra. The second term in both of Expressions No.11 and 12 is a Fresnel transmission term t_(S), and relates to thetransmission of S or P polarized light through the scanning facet by wayof the phenomenon of Fresnel transmission. The third term in both ofExpressions No. 11 and 12 is an estimated internal loss term (1−0.1),and relates to an estimate of 8% loss due to scattering and absorptionin the gelatin and 2% Fresnel reflection loss at the gelatin/glassinterfaces. Collectively, these three terms specify the lightdiffraction efficiency of the i-th scanning facet to S or P polarizedlight incident thereto.

Thus, by embracing the terms of Expressions 11 and 12, mathematicalExpression No. 13 in FIG. 10C2 is used to calculate the totalout-and-back diffraction efficiency for an S-polarized outgoing beam. Inpracticing the scanner design method of the present invention, thislight diffraction efficiency expression (Expression No. 14) is insertedin the proper cells of the spreadsheet-based Scanline Production Modelrunning on the HLD Workstation. While the S and P polarizationdiffraction efficiencies E_(S)[Δn_(i)] and E_(P)[Δn_(i)], and the totalout-and-back diffraction efficiency for an S-polarized outgoing beamT_(S)[Δn_(i)], are plotted for facet Nos. 1 and 16 in FIGS. 10E1 and10E2, respectively, for different values of modulation index Δn_(i), thespreadsheet Model in practice uses the value of modulation index Δn_(i)which maximizes T_(S)[Δn_(i)]. Once this value of Δn_(i) is found andthe maximum T_(S)[n₁] computed for each scanning facet, then theout-and-back diffraction efficiency of each i-th facet relative to facetNo. 16 (i.e. the relative light diffraction efficiency, H_(i)) iscomputed for each i-th scanning facet and stored along with the value ofΔn_(i) used to compute this parameter value. H_(i) is the relevantparameter used in the spreadsheet based Scanline Production Model of thedesign process.

Having described the case where no cross-polarizer is used before thephotodetector, it is appropriate to now consider the case when using across-polarizer before the photodetector. This technique is used tocombat glare from glossy substrates and/or overcoats. In such a case,the light diffraction efficiencies of the scanning facets on thescanning disc will be modified to accommodate the fact that light of onepolarization is diffracted by the facets during scanning, but only thereturn light of the orthogonal polarization state diffracted by thefacet will pass through the crossed polarizer to the detector. In thiscase, the light diffraction efficiency analysis to be used for computingH_(i) is described in FIGS. 10F through 10I2. In all but a few respects,the light diffraction efficiency analysis for the cross-polarizer caseis quite similar to the case without cross-polarizer. The majordifference in the analysis is that the mathematical expression for totalout and back light E_(t)[Δn] is defined as the product of the S and Plight diffraction efficiencies, rather than the product of the Sdiffraction efficiency and the average of the S and P diffractionefficiencies, as shown in Expression 13 in FIG. 10C2. In practicing thescanner design method of the present invention, this light diffractionefficiency expression (No. 14) is inserted in the proper cells of thespreadsheet-based Scanline Production Model running on the HLDWorkstation. While the S and P polarization diffraction efficienciesE_(s)[Δn_(i)] and E_(p)[Δn_(i)], and the total out-and-back diffractionefficiency for an S-polarized outgoing beam E_(t)[Δn_(i)] are plottedfor facet Nos. 1 and 16 in FIGS. 10I1 and 10I2, respectively, fordifferent values of modulation index Δn_(i), the spreadsheet Model inpractice uses the value of modulation index Δn_(i) which maximizesE_(t)[Δn_(i)]. Once this value of Δn_(i) is found and the maximumE_(t)[n₁] computed for each i-th scanning facet, then the out-and-backdiffraction efficiency of each i-th facet relative to facet No. 16 (i.e.The relative light diffraction efficiency, H_(i)) is computed for eachi-th scanning facet and stored along with the value of Δn_(i) used tocompute this parameter value. H_(i) is the relevant parameter used inthe spreadsheet based Scanline Production Model.

At Block H in 11B, the spreadsheet-type Scanline Production Modelproceeds to compute for each i-th scanning facet, the Relative LightCollection Efficiency Factor ξ_(i). Notably, this parameter is computedusing Expression No. 18 in FIG. 8C2 and the various parameter valuesspecified therein which have been previously assumed and evaluated. Inthe present invention, the Total Light Collection Efficiency of eachholographic facet is substantially the same (equal) when measured fromits focal point f_(i). As indicated in Expression No. 18, the “relative”Light Collection Efficiency factor ζ_(i) for each i-th facet comprisesthree terms: the first term is a Lambertian geometrical term; the secondterm is a projected area term; and the third term is a relative lightdiffraction efficiency term (H_(i)). The Lambertian geometry term isformulated in terms of the focal length of the facet, f_(i), and thefocal length of facet No. 16, f₁₆. The projected area term is formulatedin terms of the diffracted beam angle of the i-th scanning facet, B_(i),and the diffracted beam angle of facet No. 16, B₁₆. The relative lightdiffraction efficiency for the i-th scanning facet, H_(i),is formulatedin terms of the total out-and-back light diffraction efficiencies forthe i-th facet and the 16th facet, as described in great detail above.Notably, inasmuch as the relative light diffraction efficiency for eachi-th facet H_(i) is a function of the facet's modulation index, Δn_(i),which maximizes H_(i), the relative Light Collection Efficiency Factorfor each i-th scanning facet, ζ_(i), is also a function of themodulation index Δn_(i), a parameter which can be controllably realizedduring facet construction in the laboratory as well as on the productionline. These three terms in Expression No. 18 of FIG. 8C2 represent threecritically important design considerations necessary to construct ascanning disc, wherein each facet has substantially the same Lambertianlight collection efficiency. In the next step of the design process, itremains to be taught how this objective can be carried out while usingsubstantially all of the available surface area on the scanning disc.

Having calculated the Light Collection Efficiency Factor ζ_(i) for eachscanning facet on the disc under design, the spreadsheet-based ScanlineProduction Model proceeds to Block I of FIG. 11B where it usesExpression No. 19 in FIG. 8C2 to calculate the Total Light CollectionArea of each i-th scanning facet, Area_(i), on the scanning disk underdesign. Notably, the first term in Expression No. 19 reflects the factthat all of the available light collecting area between the outer radiusand inner radius (i.e., adjacent the disk support hub) is used inapportioning light collecting surface area to each scanning facet on thedisk. The second term in Expression No. 19 of FIG. 8C2 reflects the factthat the total light collecting surface area of each facet Area_(i) iscomputed by weighing the total light collecting surface area availableon the scanning disk by an “equalized” light collecting efficiencyfactor. As indicated by Expression No. 19 of FIG. 8C2, this “equalized”light collecting efficiency factor is computed by dividing the i-thlight collecting efficiency factor by the sum of all light collectingefficiency factors for all of the sixteen facets. Thus, each holographicfacet on the scanning disc is capable of collecting substantially thesame amount of reflected laser light and directing it onto the paraboliclight focusing mirror beneath the disk, independent of the location ofthe scanned code symbol within the scanning volume of the system. Inpractical terms, this means that each facet will focus substantially thesame amount of light onto a photodetector, independent of whether thescanned code symbol resided at the farthest focal plane or the closestfocal plane in the scanning volume.

At Block J in FIG. 11B, the scanner design uses the spread-sheet basedScanline Production Model to determine, for each facet, the minimalvalue for the facet inner radius r_(i) that allows the scanner housingheight h to be equal to the desired scanner housing height h_(desired),specified by customer requirements. This step of the design processinvolves using the optimized parameters determined above to determinethe set of inner radius parameter values, {r_(i)}, for all facets on thescanning disk which provides the desired scanner housing heighth_(desired), required by the system specifications, below which the beamfolding mirrors must be contained while ensuring the production of theprespecified scanning pattern. Before describing the reiterativeevaluation procedure used to find the set of minimum inner radiusparameter values {r_(i)} which satisfy the necessary conditions toensure that h=h_(desired), it will be helpful to first describe how theinner radius r_(i) for each facet can be found in terms of othergeometrically related parameters in the system.

As illustrated in FIG. 8A, the angle (i.e. B−β) in FIG. 8A1 of the raygoing to the innermost part of the light collection portion of each i-thfacet is calculated using the ray projected from the maximum readingdistance point at the center of each scanline to the inner radius of thei-th facet on the virtual scanning disk. The intersection of this rayand the beam folding mirror is used to establish the height of thefolding mirror, y_(j). Notably, only the ray giving the maximum mirrorheight is used to set the final mirror height. As described inExpression No. 11 in FIG. 8C1, this dimension y_(j), plus the dimensiond beneath the disk for the light collection optics, establishes theoverall height of the scanner housing, h.

The tilt angle of the beam folding mirror φ_(j) is one of the parametersthat can be varied (i.e. assumed) to arrive at a “best” scanner design.It has been found that a large tilt angle (away from the scan beams)results in a shorter housing size, but requires very shallow exit anglesfor the beams leaving the holographic scanning disk. This makes thescanning disk difficult to fabricate and lowers the overall lightdiffraction efficiency and thus total light collection efficiencythereof. It also results in unnecessarily high beam speeds. A small tiltangle will result in better exit angles for the beam leaving theholographic disk, but results in a taller scanner housing size and areduction in the scan lengths of the scan lines for the 16 facetscanning disk of the illustrative embodiment. After severalreiterations, an optimum tilt angle φ_(j) for the beam folding mirrorwas established at 16 degrees from the vertical.

In the reiterative evaluation procedure used to find the minimum r_(i),the goal is essentially to minimize r_(i) for all facets as this willensure that the maximum amount of available light collecting space onthe scanning disc is utilized for light collection. If the inner radiusparameter r_(i) for each facet is minimized while all other conditionsare being satisfied, then the amount of laser light reflected off thescanned symbol and collected by the facets on the rotating scanning discwill be maximized, thereby producing strong scan data signals at thephotodetectors of the system. Also, as shown by Expressions No. 11 and10 in FIG. 8C1, minimizing r_(i) for each scanning facet causes theheight of the beam folding mirrors to be greater, necessitating ascanner housing with an increased height dimension. Thus, adjustment ofthe inner radius of the facets has significant effects on otherimportant geometrical parameters in the holographic scanning system.

In general, the reiterative evaluation procedure supported by thespreadsheet-based Scanline Production Model typically comprises a numberof design cycles, each of which can be identified by an assigned cycleindex k=1,2,3,4,5, . . . ,6,7,8, etc. During the (k=1)th cycle, the discdesigner uses Equations No. 4 through 11 in FIG. 8C1 to compute the beamfolding mirror height, h. In order to compute an initial value for h(i.e. h_(i)) using an initial value for each r_(i), an initial value foreach r_(i) (e.g. 1.0 inch) is selected for the first run of calculations(e.g. r₁=1.0, r₂=1.0, . . . , r₁₆=1.0). The result of this cycle ofcomputations is a set of scanner housing height values, (e.g. h₁=12.0inches, h₂=12.5 inches, . . . , h₁₅=15.0 inches, h₁₆=12.3 inches) whereh₁₅=15.0 inches in the illustrative example is the maximum heightcomputed for the initial value for each r_(i).

If none of the computed height values are equal to or below h_(desired),then during the (k+1) cycle each inner radius parameter r_(i) isincremented by a very small amount (e.g. +0.2 inch) and the scannerheight parameter hi is recalculated for each value of facet inner radiusr_(i). The set of scanner height values are then analyzed by the scannerdesigner to determine which values of r_(i) yielded scanner housingheight (h_(i)) values less than or equal to h_(desired). Each value ofr_(i) that yielded a scanner housing height h_(i) value less than orequal to h_(desired), is stored in memory of the HSD workstation andfixed in subsequent computational cycles of the reiterative process.Each value of r_(i) that did not yield a scanner housing height h_(i)value less than or equal to h_(desired), is changed in subsequentcomputational cycles of the reiterative process.

If all of the computed height values are equal to or above h_(desired),then during the (k+1) cycle, then each inner radius parameter r_(i) isincremented by a very small amount (e.g. +0.2 inch) and the scannerheight parameter h_(i) is recalculated for each value of facet innerradius r_(i). The set of scanner height values are then analyzed by thedisc designer to determine which values of r_(i) yielded scanner housingheight (h_(i)) values less than or equal to h_(desired). Each value ofr_(i) that yielded a scanner housing height h_(i) value less than orequal to h_(desired), is stored in memory and fixed in subsequentcomputational cycles of the reiterative process. Each value of r_(i)that did not yield a scanner housing height h_(i) value less than orequal to h_(desired), is changed in subsequent computational cycles ofthe reiterative process.

The reiterative evaluation process progresses as described above until avalue for each inner radius r_(i) is found which yields a scannerhousing height h_(i) which is less than or equal to the desired scannerhousing height h_(desired). When this point in the process is reached,then the spreadsheet-based Scanline Production Model will havedetermined a set of inner radius parameter values {r_(i)} for the facetson the scanning disk under design.

At Block K in FIG. 11B, the spreadsheet-based Scanline Production Modeluses the assumed value for r_(outer), the optimized set of parametervalues {r_(i)}, and the previously computed set of light collectionefficiency values {ζ_(i)}, to compute the net light collection surfacearea for each i-th scanning facet, Area_(i), such that each and everyfacet collects at its photodetector substantially the same amount oflight from its corresponding scanline, while substantially all of thesurface area available on the scanning disc is utilized for lightcollection purposes. In order to ensure that such conditions aresatisfied during this set of parameter computations, Expression No. 19in FIG. 8C1 includes mathematical structure which defines a term forsurface area computation which in conjunction with the proportionedhologram efficiency factor (i.e. ζ_(i)/Σ(ζ_(i))), will provide lightcollection efficiency equalization (i.e. normalization). Upon completionof this step, a set of facet surface areas {Area_(i)} is produced.

At this stage of the process, the spreadsheet-based Scanline ProductionModel holds for each facet a set of geometrical parameters which, intheory, would be sufficient to construct a scanning disc capable ofproducing the prespecified scanning pattern during the initial stage ofthe design process. Specifically, this proposed set of geometricalparameters comprises: a set of facet Rotation Angle values {θ′_(roti)}for the holographic facets; a set of inner radius values {r_(i)} for theholographic facets; a set of Total Light Collection Surface Areas{Area_(i)} for the holographic facets; a set of Focal Length values{f_(i)} for the holographic facets; and a set of modulation index values{Δn_(i)} for the holographic facets. Collectively, these parametersshall be referred to as “construction parameters” as they are used toconstruct the facets on the holographic scanning disk. Notably, thesubset of construction parameters {θ′_(roti),r_(i),Area_(i)} provides ageometrical specification for the i-th scanning facet which, in general,has irregularly shaped boundary characteristics constrained by theseconstruction parameters and the condition that all of the availablesurface area on the disk be utilized for light collection.

Having found a set of facet parameters which will produce theprespecified laser scanning pattern, while satisfying scanner housingdesign constraints, it nevertheless is essential to determine whetherthe set of facet construction parameters, derived from the scannerdesign process, can be physically laid out on the available surface areaof the scanning disk whose geometry has been previously bounded by outerradius r_(outer).

During the facet lay out verification stage of the design processindicated at Block L of FIG. 11B, the scanner designer tries tophysically layout on the surface of the scanning disc, each of thegeometrically-specified holographic facets in a facet order which allowsmaximum use of the disk surface area. Inasmuch as each facet has been“loosely” constrained by its construction parameters{θ′_(roti),r_(i),Area_(i)}, the disk layout designer is accorded adegree of freedom in which to specify the perimetrical boundaries ofeach facet so that substantially all of the available surface area onthe disk is occupied by the facets, while the construction parameters{θ′_(roti),r_(i),Area_(i)} for each i-th facet are satisfied. When thedisk layout designer has achieved this objective, then the complete setof construction parameters{θ′_(roti),r_(i),Area_(i),f_(i),Δn_(i),A_(i),B_(i)} for i=1,2, . . . ,16can be used to make the designed scanning disk.

In the preferred embodiment, a geometrical modeling tool, such asAUTOCAD, supported by the HSD workstation is used to geometrically modeleach scanning facet and layout the same on the scanning disk whilesatisfying several global constraints, namely: (1) that substantiallyall of the light collecting surface area available on the scanning diskis utilized; (2) that at the end of each scanline sweep, all or almostall of the light collection surface area associated with thecorresponding facet is disposed immediately above the paraboliccollection mirror (i.e. light collection element) to maximize lightdetection at the photodetector; and (3) that all incoming light raysreflected from a scanline produced by the j-th scanning station, strikeits associated beam folding mirror and are collected by the samescanning facet which produced the scanline, to avoid signal clipping andthus ensuring maximal SNR at the photodetector. Notably, during thisstage of the scanner design process, a set of construction parameters{θ′_(roti),r_(i),Area_(i)} for all values of i cannot be changed oraltered for any of the holographic facets, but rather must be maintainedas constants throughout the procedure. Specifically, the facet layoutprocedure is carried out by adjusting the boundary lines for each facet,while satisfying the above described constraints and facet parameters{θ′_(roti),r_(i),Area}.

If the scanner designer can successfully layout the facets on the discusing the tools available within the HSD workstation, then the discdesigner proceeds to the final stage of the design process indicated atBlock M where designed scanner is analyzed against its designperformance criteria (e.g. equalized light collection efficiency amongthe facets, etc.). This stage of the process is carried out usingvarious analytical tools available in the HSD workstation. For example,the HSD workstation provides the scanner designer with a tool forcomputing the Lambertian light collection efficiency, EL, of each faceton a designed scanning disc. The purpose of this tool is to allow thescanner designer to quickly compute the Lambertian light collectionefficiency of each i-th facet on a designed scanning disc, and determinewhether such light collection efficiency measures are substantiallyequal for each facet on the designed scanning disc. If not, then thescanner designer can return to the spreadsheet-based Scanline ProductionModel and modify the disc and/or scanner design until acceptableperformance parameters are obtained for the application at hand. Below,the structure and function of the Lambertian light collection efficiencymeasuring tool will be described in greater detail.

In FIGS. 10J through 10L1, a geometrical optics model (I.e. LambertianRadiator Model) is presented for calculating the Lambertian lightcollection efficiency, E_(L), of each i-th facet on a scanning discproduced using the disc design procedures of the present invention. Theparameters associated with the Lambertian Radiator Model aregeometrically defined in FIG. 10K. The set of equations listed in FIG.10L1 define relationships among certain of the parameters in the model.Notably, the E_(L) calculation procedure described herein does notinclude factors related to diffraction efficiencies, holographic disktransmission characteristics for off-Bragg angles, mirror reflectances,window transmission characteristics and bar code label reflectances. Itis understood that all of such parameters must be taken into account todetermine the total light collection efficiency of the scanning system.As these miscellaneous factors have been previously discussedhereinabove, modifications to the present procedure to improve itsdegree of accuracy will readily occur to those skilled in the art.

The geometrical optics model of FIG. 10J assumes that most bar codesymbol surfaces behave as Lambertian radiators, wherein the process ofirradiance from such surfaces (i.e. “diffusely reflective surfaces”) isgoverned by Lambert's Law during laser beam scanning and lightcollection operations. In accordance with Lambert's Law, laser lightdiffusely reflected off the scanned code symbol is projected over anarea having a circular collection aperture (i.e. A_(circular)). In orderto calculate the Lambertian light collection efficiency E_(L) of eachi-th facet, Lambert's Law requires that each facet have a circulargeometry. In general, each facet on the scanning disc of the presentinvention has a non-circular geometry. To allow the use of theE_(L)-calculation procedure on such scanning disc, it is thereforenecessary to first compute an effective circular aperture, A_(eff), foreach i-th facet on the scanning disc under testing. This equivalentmeasure can be easily computed using the previously determined surfaceArea_(i) of the i-th facet and the well known circle-area formula (i.e.Area_(i)=πR²=A_(eff)), where R is defined as the radius of the effectivecircular aperture for the facet.

As shown in FIG. 10J, the Lambertian radiator model comprises a numberof other geometrical parameters which factor into the calculation ofE_(L) namely: Z, the distance from the point of code scanning to theeffective circular aperture defined on the scanning disc; R_(pr), theradius of the projected effective circular aperture; B_(i), thediffraction angle of the outgoing laser beam from the i-th facet; andδ_(i), the half-angle subtended by the effective projected circularaperture. During the measurement stage of the procedure, physicalmeasurements are made to determine Z_(i) (in inches), Area_(i) (insquare inches) and B_(i) (radians) for each i-th facet. Compute A_(eff)using the formula A_(eff)=A_(i) Sin (B_(i)). Then R_(pr) is computedusing the circle area formula: A_(eff)=πR_(pr)2. Then using computedR_(pr) and measured Z_(i), the half-angle δ_(i) for the i-th facet iscomputed using the expression: δ_(i)=a Tan [R_(pr)/Z_(i)], wherein atan=tan⁻¹. Having computed δ_(i), E_(L) can be calculated using theexpression E_(L)=[sin(δ_(i))]² for small values of δ_(i) (i.e., lessthan 2 degrees). In FIG. 10L1, a numerical example is worked out forillustrative purposes.

Ideally, each facet should have equal Total Light Collection Efficiencywhich is defined as E_(Li)·H_(i), for all values i. In mostapplications, one can expect the Total Light Collection Efficiency ofthe facets to deviate within an acceptable tolerance range, yet stillconsider such a scanning disc to have the total light collectionefficiency of its holographic facets substantially “equalized” withinthe spirit of the present invention.

When the scanner designer determines that the scanning disc designsatisfies its design criteria (e.g. equalized light collectionefficiency among the facets, etc.), then the disc design process iscompleted and the facets of the scanning disc can be manufactured andthereafter assembled between the glass support plates of the disc.However, if the scanner designer cannot successfully layout the facetson the disc as described above, then as indicated at Block M in FIG.11C, the designer may return to any of the stages in the scanner designprocess, and use the spreadsheet-based Scanline Production Model torecompute parameters based on newly assumed parameters in the scannermodel. During this interactive design process, the scanner designer canperform “what-if” type analysis in order to arrive at a best or mostsuitable scanner design, given the set of system constraints presentedto the designer.

Designing a Holographic Laser Scanner Having Cross-Polarizing Filtersbefore Its Photodetectors

At this juncture it is appropriate to now describe how to design aholographic scanning disk for use in a holographic laser scanneremploying light polarization filtering.

As shown in FIG. 10F, the S (or P) polarized laser beam produced fromeach VLD in the system is directed incident the scanning disk,sequentially diffracted by the rotating holographic facets, and thenreflected off the beam folding mirrors towards a bar code symbol to bescanned within the scanning volume. As is well known, a portion of the S(or P) polarized laser beam incident on the code symbol is reflected offthe glossy surface (i.e. substrate or overcoat) as an optical signalwhich retains the polarization state of the incident laser beam. Theother portion of the polarized laser beam passes through the glossycoating, is intensity modulated and scattered (i.e. diffused) by thecode symbol and reflects off the symbol as an unpolarized,intensity-modulated optical signal. A portion of these two signalcomponents collectively travel back along the same optical path as theincident scanned laser beam and is diffracted by the corresponding facettowards the parabolic mirror. The parabolic mirror focuses the collectedlight rays of the reflected laser beam through the same facet withminimal light diffraction (i.e. off Bragg) through a P (or S) polarizingfilter which attenuates (i.e. blocks) the S (or P) polarized componentof the scan data signal while transmitting the P (or S) polarizedcomponent of the unpolarized component thereof to the photodetector forintensity detection. Advantageously, when using this scanningarrangement, the S (or P) polarized 0-th diffractive order of the laserbeam incident the facet is also blocked by the cross-polarizing filter,thus improving the SNR of the detected scan data signal in general. Asshown in FIG. 10F, and as used hereinafter, the term S-cross polarizingfilter shall mean a polarized light and block P polarized light, whereasthe term P-cross polarizing filter shall mean a polarizing filteroriented on a photodetector so as to pass P polarized light and block Spolarized light.

While the use of an S or P cross-polarizing filter effectively solvesthe problems associated with glare in the holographic scanning systemdescribed above, it does require a minor modification of the scannerdesign process of the present invention. In particular, the lightdiffraction efficiencies of the scanning facets must be modified fromthe way taught in Expressions No. 11 through 13 of FIG. 10C2 due to thefact that light of one polarization must be efficiently diffracted bythe facets during scanning, while light of the orthogonal polarizationmust be efficiently diffracted by the holographic facet during lightcollection and detection. This condition is achieved by ensuring thatthe product of the outgoing S (or P) polarization diffraction efficiencyof each facet and the return P (or S) polarization diffractionefficiency is maximized. Thus, the total out-and-back diffractionefficiency of each i-th facet, H_(i), is defined as the “product” of (i)the outgoing diffraction efficiency of the facet for the S (or P)polarization component of the incident laser beam and (ii) the returndiffraction efficiency of the facet for the orthogonal P (or S)polarization of the laser beam.

When designing each facet on the scanning disc, all of the steps in thedisc design method recited at Blocks A through F in FIG. 11A are carriedout in the manner described above. The only modification to the scannerdesign method occurs at Block G of FIG. 11B when determining theholographic diffraction efficiency H_(i) for each facet. At this stageof the method, Expression No. 13 set forth in FIG. 10H2 is used tocompute E_(t)[Δn_(i)], the total out-and-back diffraction efficiency ofeach i-th facet, H_(i). As indicated by this mathematical expression,this parameter is defined as the product of the S and P lightdiffraction efficiencies, rather than the product of the S (or P)diffraction efficiency and the average of the S and P efficiencies, asshown in Expression 13 in FIG. 10C2, namely: E_(S)[Δn_(i)], the outgoingdiffraction efficiency of the facet for the S polarization component ofthe incident laser beam; and E_(P)[Δn_(i)], the return diffractionefficiency of the facet for the orthogonal P polarization of the laserbeam. These individual diffraction efficiency terms are provided byExpressions No. 11 and 12, respectively, in FIG. 10H2. As indicated inFIG. 10H2, component terms E_(S)[Δn_(i)] and E_(P)[Δn_(i)] and productterm E_(t)[Δn_(i)] are graphically plotted as a function of modulationindex Δn_(i) of the recording emulsion from which the i-th holographicfacet is realized.

To practice the scanner design method of the present invention, thislight diffraction efficiency expression (14) is inserted in the propercells of the spreadsheet-based Scanline Production Model running on theHLD Workstation. While the S and P polarization diffraction efficienciesE_(S)[Δn_(i)] and E_(P)[Δn_(i)], and the total out-and-back diffractionefficiency for an S-polarized outgoing beam E_(t)[Δn_(i)] are plottedfor facet Nos. 1 and 16 in FIGS. 10I1 and 10I2, respectively, fordifferent values of modulation index Δn_(i), the spreadsheet based Modelin practice uses the value of modulation index Δn_(i) which maximizesE_(t)[Δn_(i)] for the i-th facet. Once this value of Δn_(i) is found andthe maximum E_(t)[n₁] computed for each i-th scanning facet, then theout-and-back diffraction efficiency of each i-th facet relative to facetNo. 16 (i.e. the relative light diffraction efficiency, H_(i)) iscomputed for each i-th scanning facet and stored along with the value ofΔn_(i) used to compute this parameter value. This computation is carriedout for each of the sixteen facets on the scanning disc under design.Then, the relative holographic diffraction efficiency H_(i) for eachi-th facet is computed as the ratio of the product termsE_(t)[Δn_(i)]/E_(t)[Δn₁₆]. After carrying out this cycle ofcomputations, a set of relative diffraction efficiencies {H_(i)} areobtained for the scanning disc that has been particularly designed foruse with cross-polarization filters. Thereafter, the scanner designerreturns to the spreadsheet-based Scanline Production Model to Block Hand resumes the scanner design process described hereinabove untilcompletion.

Designing a Holographic Laser Scanning Disc Having Facets with DifferentFringe Contrast over the Beam Scanning and Light Collecting PortionsThereof, for Use in a Holographic Scanning System with Light PolarizingFilters

In the above-described embodiment of the scanner design method hereof,cross-polarizers were used to eliminate the effects of glare duringscanning. In the scanning disc design described above, the S and Pdiffraction efficiencies E_(S)[Δn_(i)] and E_(P)[Δn_(i)] were not bothmaximized, but rather the product of these terms, namely E_(t)[Δn_(i)],was maximized by finding the modulation index Δn_(i) at which thisfunction “peaked” or attains its maximum value, assuming that themodulation index of the fringe structure throughout the i-th facet isuniform or the same over the entire facet. This fact is illustrated inthe diffraction efficiency plots of FIG. 12. Notably, this designtechnique offers a compromise to the problem at hand by accepting thefact that the light diffraction efficiencies of the facets to S and Plight do not attain maximum or peak values at the same value ofmodulation index, Δn_(i).

In the alternative disc design described hereinbelow, this constraint isremoved from the design process, and instead of finding the singlemodulation index value for the emulsion at which the product of thediffraction efficiencies E_(S)[Δn_(i)] and E_(P)[Δn_(i)] are maximized,the alternative technique finds the modulation index value Δn_(i1) atwhich E_(S)[Δn_(i)] is maximized (e.g. peaked) and the modulation indexvalue Δn_(i2) at which E_(P)[Δn_(i)] is maximized. Then during the facetmanufacturing process, the i-th facet is selectively exposed to achieveportions with different light diffraction deficiencies, namely: theemulsion of the facet at the outer portion of the i-th facet along whichthe incident laser beam is incident is exposed by the Argon laser beamso that modulation depth Δn_(i1) is attained and thus light diffractionefficiency E_(S)[Δn_(i1)] maximized; and the emulsion of the facet atthe inner portion of the i-th facet along which the rays of the returnlaser beam pass for collection is exposed by the construction laser beam(e.g. an Argon laser) so that modulation depth Δn_(i2) is attained andthus light diffraction efficiency E_(P)[Δn_(i1)] maximized. A scanningdisc of such a design is shown in FIG. 12A. During this two-stepexposure process, spatial masks are used to cover the regions of thei-th facet which are not to be exposed during the particular exposureprocess. By carrying out this facet design and construction technique, ascanning disc is produced having facets which optimize the diffractionefficiency of the S-polarization component of the laser beam incidentthe rotating scanning disc during scanning operation, while optimizingthe diffraction of the P-polarization component of the laser beamreflected off a scanned symbol during light collecting operations.

As will be readily apparent, the use of a scanning disc having facetswith regions made from emulsions (i.e. DCG) characterized by differentdepths of modulation, Δn_(i1) and Δn_(i2), will provide the holographiclaser scanner of the present invention with a better overall lightcollection efficiency, as the inner light collecting portion of thefacet does not have to be exposed to maximize the product of theefficiencies of the S and P polarizations, but rather exposed tomaximize the efficiency for the polarization of the return light raysthat are passed to the photodetector by the cross-polarizer thereon.This feature of the present invention will result in a significantimprovement in the light collection efficiency of facets having largediffraction angles (or small B) (i.e. Nos. 4,8,12, and 16). Theimprovement which can be expected when using this technique is about 50%average improvement, which is the difference between using a 30milliwatt laser beam over a 20 milliwatt laser beam, or a 40:1 SNRversus a 30:1 SNR. This provides a markedly improved performance whenreading code symbols printed on glossy substrates or having glossyovercoats, as in many stock products.

As shown in FIGS. 12B1 through 12B3, a modified procedure is providedfor designing a holographic laser scanner employing a scanning dischaving dual modulation-depth or fringe-contrast regions over the beamsweeping and light collecting regions thereof. As shown, the steps ofthe method indicated at Blocks A though F and J through N in FIGS. 12B1through 12B3 are substantially the same as in the method described inFIGS. 11A through 11C. The points of difference between the twoalternative design methods begin at Block G in FIG. 12B2 where thespreadsheet-based Scanline Production Model running on the HSDworkstation computes an “effective” relative light diffractionefficiency factor H′_(effi) for each i-th split-design facet on thedisc. The mathematical expression set forth in FIG. 12C is used tocompute the parameter H_(effi) for each scanning facet. As indicated bythis expression, a number of dependent parameters are involved in thiscalculation, including a number of area terms which must be initiallyassumed to perform the calculation. Other terms, such as the lightdiffraction efficiencies E_(S)[Δn_(i1)] and E_(P)[Δn_(i2)] for each i-thfacet can be computed using the expressions for light diffractionefficiency set forth in FIG. 10H2. The outer area of the i-th facetA_(outeri) can be assumed using knowledge of the laser beam diameter andthe facet Rotation angle θ_(roti), Expression No. 17 in FIG. 8C2 of thefacet, whereas the inner area of the facet A_(inneri) can be calculatedby subtracting the inner area A_(inneri) from the total area of thefacet, A_(totali). For purposes of the design method, the parameterA_(totali) is assumed to be the Area_(i) provided by the design methodof FIGS. 11A through 11C.

After H_(effi) is calculated, the scanner designer proceeds to Block Hand uses the spreadsheet-based Scanline Production Model to compute thelight collection efficiency factor ζ_(i) for each facet. Then at BlockI, the scanner designer uses the spreadsheet-based Scanline ProductionModel to compute the total light collection surface area of the facet,A_(totali). At Block I′, the scanner designer uses the spreadsheet-basedScanline Production Model to compare the assumed values of A′_(inneri)with computed values of A_(totali). Then based on the differencesbetween these parameter values, the scanner designer returns to Block Gin the design method, adjusts the assumed values for A′_(totali) andthen repeats the steps indicated at Blocks G through I′, each timeyielding a different value for H_(effi) required in the total areacomputation for the i-th scanning facet. When A′_(totali) converges uponA_(totali), then acceptable values for H_(effi) and A_(totali) have beenfound and the design process can then proceed to Block J and resume inthe manner described in connection with FIGS. 11A through 11C. When anacceptable set of geometrical parameters have been obtained whichsatisfy the specified system constraints and performance criteria, thedesign process is completed and the scanner design can be constructed.

Conversion of Scanning Disc Reconstruction Parameters

Typically, there is a great need to mass manufacture the holographicscanning disc in very large numbers. Thus, holographic masteringtechniques are ideally used. While any suitable mastering technique canbe used, it will be necessary in nearly all instances to holographicallyrecord the master facets at a recording wavelength λ_(C) which isdifferent than its reconstruction wavelength λ_(R). The reason for thisis generally well known: it is difficult to make holographic facets withhigh fringe-contrast at the reconstruction wavelength λ_(R), which inthe illustrative embodiment is about 670 nanometers. Instead, it iseasier to record the facets at a spectral wavelength at whichhigh-contrast fringes can be realized and then play back at thewavelength of the VLDs in the scanner.

Presently, the preferred recording medium for recording facets withhigh-contrast fringes is Dichromated Gelatin (DCG) which exhibits itsgreatest sensitivity near 488 nm. Thus a blue laser beam is requiredduring recording. In order to record the i-th HOE at its constructionwavelength, and then reconstruct the same at another wavelength, it isnecessary to translate (i.e. convert) its construction parameters{f_(i),A_(i),B_(i)} expressed at the reconstruction wavelength λ_(R),into a complete corresponding set of parameters expressed at thespecified construction wavelength λ_(C). The process illustrated inFIGS. 28A1 through 28D can be used to carry out the necessary parameterconversions. In addition, non-symmetrical optical elements areintroduced to eliminate, or minimize, aberrations produced by thewavelength shift between exposure and reconstruction using techniqueswell-known in the art. Thereafter, using the converted set ofconstruction parameters, the HOE facets can be made using the convertedset of construction parameters and the holographic recording systemschematically represented in FIG. 13.

In FIGS. 28A1 and 28A2, a geometrical optics model is schematicallypresented for an incident laser beam being deflected by a facetholographic optical element (HOE), e.g. realized as a volume-typetransmission hologram supported on the rotating scanning disc. As shownin FIG. 28A1, the incident laser beam enters the upper glass plate ofthe disc at incident angle θ_(i) (i.e. 90°−A_(i)), propagates throughthe upper glass plate, the gelatin, the lower glass plate, and thenemerges therefrom at diffraction angle θ_(d) (i.e. 90°−B_(i)) towardsits associated beam folding mirror. As indicated in FIG. 28A2, the laserbeam being transmitted through the disc plates and gelatin of theholographic facet interacts with the high-contrast fringes recordedtherein so that its direction of propagation is changed (i.e. modified)through the process of diffraction physics. As shown in these drawings,a number of parameters are required to construct a suitable geometricaloptics model for this laser beam diffraction process, and the process bywhich the construction parameters are converted. In general, there aresix input parameters to the conversion process and two outputparameters. Three of the input parameters are derived from the scanningdisk design process, namely: λ₁, the wavelength of the laser beamproduced by the VLD during hologram reconstruction (i.e. laser beamscanning); the incident angle θ_(i.1) (i.e. 90°−A_(i)) at which thelaser beam propagates through the facet (i.e. upper glass plate, thegelatin, the lower glass plate) during reconstruction (i.e. laserscanning); and the diffraction angle θ_(d.1) (i.e. 90°−B_(i)) at whichthe diffracted laser beam emerges from the facet and propagates towardsto its associated beam folding mirror. The other three input parametersprovided to the parameter conversion process are derived from the HOEconstruction technique used to fabricate holographic facets, namely; λ₂,the wavelength of the laser beam used during HOE construction; n₀, theaverage (i.e. bulk) index of refraction of the recording medium beforefringe development processing; and n₂, the average index of refractionof the recording medium after fringe development processing.

As set forth in the table of FIG. 28AD, the conversion process producestwo output parameters, namely: θ_(i.2), the Angle of Incidence(Reference Beam Angle) θ_(R) for the second (construction) wavelengthλ_(C), and θ_(d.2), the Angle of Diffraction (Object Beam Angle) θ_(O)for the second (construction) wavelength λ_(C), both defined in FIG. 13.These two parameters and the aberration correcting optics are used toconfigure the HOE recording system shown in FIG. 13. All otherparameters comprising the process model are intermediate parametersinasmuch as they establish relationships between the input and outputparameters of the conversion process. In FIGS. 28, 28C1 and 28C2, theseintermediate parameters are defined as follows: the incident angle α_(i)inside the medium after development processing; the incident angle β₁inside the medium after processing; d, the surface inter-fringe spacingof the recorded fringes; φ, the tilt angle of the Bragg Planes; θ_(0.1),the Angle relative to the Bragg planes; L, the separation of the Braggplanes, determined by the Bragg condition equation; θ_(0.2), the Anglerelative to the Bragg planes for the second (i.e. construction)wavelength satisfying the Bragg condition, before fringe developmentprocessing; α₂, the Angle of Incidence inside the recording medium forthe second wavelength, before fringe developing processing; and β₂, theAngle of Diffraction inside the recording medium for the secondwavelength, before fringe developing processing.

Using the input parameters defined above, the output parametersθ_(i.2)=θ_(O) and θ_(d.2)=θ_(R) can be readily computed using EquationsNo. 10 and 11 set forth in FIGS. 28C1 and 28C2. These two computedparameters, along with the previously determined index modulation Δn_(i)and the aberration correcting optics can be collectively used toconstruct the i-th facet of the designed scanning disc using a laserbeam having wavelength λ_(C) and a recording medium having averageindices of refraction n₀ and n₂ before and after fringe structuredevelopment, respectively. In the illustrative embodiment, the preferredrecording medium is dichromated gelatin (DCG) having its maximum lightsensitivity in the blue spectral range, and thus the necessaryconstruction wavelength for exposing this recording medium can beproduced by an Argon gas laser with a peak spectral output centered atabout 488 nanometers. For each designed facet, a set of constructionparameters are determined using the above-described method andthereafter used to physically construct a “master” facet at the second(construction) wavelength λ_(C). The master facet can then be used tomake one or more facet “copies” for mass production of the holographicscanning disk.

Constructing a Holographic Laser Scanning Disc UsingWavelength-Converted Construction Parameters

As shown in FIG. 13, each holographic facet is made by producing areference laser beam from a laser source. By passing the reference laserbeam through a beam splitter, an object laser beam is produced in aconventional manner and using anamorphic optics, an object beam isformed having beam characteristics which are specified by parametersf_(i) and θ_(ri). Then as shown, both the reference beam and the objectbeam are directed incident upon a holographic recording medium (e.g.DCG) supported upon a substrate. The angle of incidence for thereference beam is specified by parameter θ_(i2), whereas the angle ofincidence for the object beam is specified by the parameter θ_(d2), asshown. The geometrical configuration of this recording system is shownin FIG. 13 with all of the holographic facet recording parametersillustrated.

Post Manufacture Parameter Verification

After constructing a holographic scanning disc in accordance with theteachings herein disclosed, it will be desired in many applications toverify that such scanning discs in fact embody the various features ofthe present invention. Inasmuch as the particular value of modulationindex required for each facet is controlled by controlling laser powerand gelatin quality during facet exposure, there is a degree ofvariability in facet light collection efficiency which can be expectedin manufactured scanning discs. Also, inasmuch as it is impossible tomaintain a perfect degree of uniformity in thickness in the emulsionlayer of each facet during recording (i.e. exposure) operations, it canalso be expected that the light collection efficiency of each facet maydeviate slightly from its value determined during the disc designprocess. Consequently, there is a need during scanning disc manufactureto maintain accurate control over (i) the specified index modulation foreach of the facets, as well as (ii) the uniformity of the emulsionlayers of each of the facets. In order to maintain high quality controlduring the disc manufacturing process, it will be important to verifythat the light collection efficiencies of the facets on eachmanufactured scanning disc are substantially equal in value, therebyallowing the use of low band-width photodetection and signal processingcircuitry. The tool for computing Lambertian light collectionefficiency, E_(L), illustrated in FIGS. 10J through 10L and describedabove, can be used to determine that the Total Light CollectionEfficiency (i.e. E_(Li)·H_(i)) of each facet on a manufactured scanningdisc is substantially equal, as desired in nearly all holographicscanning applications.

Laser Beam Production Module of the First Illustrative Embodiment

Having described the overall system architecture of the scanner of thepresent invention and how to design and manufacture scanning disks foruse in the same, it is appropriate at this juncture to now describe ingreat detail several different embodiments of the laser beam productionmodule of the present invention, as well as different methods ofdesigning and constructing the same.

In FIG. 14, the laser beam production module of the first illustrativeembodiment is shown installed within the holographic laser scanner ofthe present invention using a parabolic light collecting mirror disposedbeneath the scanning disc at each scanning station provided therein. InFIG. 14A, the ray optics of such a scanning system are schematicallyillustrated. Notably, the laser beam production module has severalfunctions. The module should produce a circularized laser beam that isdirected at point r_(o) on the rotating scanning disk, at theprespecified angle of incidence θ_(i) (i.e. 90°−A_(i)), which, in theillustrative embodiment, is precisely the same for all facets thereon.Also, the module should produce a laser beam that is free of VLD-relatedastigmatism, and exhibits minimum dispersion when diffracted by thescanning disk.

In the first illustrative embodiment shown in FIGS. 15A through 15K, themodule 13A comprises an optical bench 60 having several adjustablemechanisms for mounting components such as a VLD 53A (53B, 53C), anaspheric collimating lens 61, a prism 62, a mirror 63 and a lightdiffractive grating 64 having a fixed spatial frequency. Thesecomponents are configured in such a manner so as to achieve the objectsof the present invention. Prior to describing how to make and assemblethe components of this module, it will be helpful to first describe thegeneral structure of each of these basic components, including theadjustable mounting mechanisms provided by the optical bench thereof.

As shown in FIG. 15, the laser beam production module of the firstillustrative embodiment is mounted beneath the edge of the paraboliclight focusing mirror, and below the associated beam folding mirror. Asshown in FIG. 15A, the optical bench of the module comprises a pointplate with a rotatable platform for mounting the prism and an adjustablesubassembly for mounting the VLD and aspheric collimating lens as anintegrated subassembly. The geometrical characteristics of prism 62 areillustrated in FIGS. 15I1 and 15I2, whereas the geometricalcharacteristics of mirror 63 and HOE plate 64 are shown in FIGS. 15J and15K, respectively. As will become apparent hereinafter, the function ofthese adjustable platforms is to allow geometrical parameters definedamong the optical components to be configured in a manner that resultsin beam circularization, astigmatism elimination, and beam dispersionminimization. The optical bench of the beam production module is mountedrelative to the optical bench of the scanning system so that theproduced laser beam is directed incident the scanning disk, at angleA_(i) defined hereinabove.

As shown in greater detail in FIG. 15B, each module bench comprises abase portion 65, and an integrally formed grating/mirror support portion66. As shown in FIG. 15C, the grating/mirror support portion 66 isdisposed at an obtuse angle relative to the base portion so that thelight diffractive grating 64 will be automatically oriented with respectto the scanning disc at a prespecified angle (determined during themodule design method hereof) when the module bench 60 is mounted onscanner bench 5, such alignment is achieved by way of pins 67 on scannerbench 5 receiving alignment holes 68 formed in the underside of modulebench 60, as shown in FIGS. 15 and 15A. The grating/mirror supportportion 66 includes a side support surface 69 for supporting the planarmirror 63, and also a top support surface 70 for supporting the lightdiffractive grating (i.e. HOE plate). Grooves can be formed along thesesupport surfaces in order to securely retain the mirror and the HOEplate.

As shown in FIG. 15B, the base portion also has a recess 71 within whichpivot plate 72 is pivotally mounted from pivot point 72, identified inFIG. 15B. As shown in FIGS. 15E1 and 15E2, pivot plate 72 has a firstportion 72A upon which a cylindrical platform 73 is rotatably mounted,and a second portion 72B upon which VLD and aspheric lens mountingassembly is fixedly mounted. The function of cylindrical platform 73 isto provide a mounting surface for the prism. Any suitable adhesive canbe used to secure the prism upon the top surface of platform 73. Anadjustment screw can be provided adjacent to the platform so that thecylindrical disk can be secured in position when adjustment of the prismhas been completed.

Subcomponents comprising the VLD and collimating lens mounting assemblyare shown in FIGS. 15E1 through 15H2. As shown in FIG. 15F1 and 15F2, aVLD mounting yoke 75 is provided for pivotally supporting an opticstelescopic assembly comprising the VLD block 76 shown in FIG. 15G1 and15G2, and the lens barrel 77 shown in FIG. 15H1 and 15H2. The functionof the VLD block 76 is to securely mount the VLD at one end thereof. Thefunction of lens barrel 77 is to securely retain the asphericcollimating lens 61. A spring is located between the VLD housing andlens barrel for producing a resistive force against the threading actionof the lens barrel while adjusting the VLD-to-lens distance parameter.Also, this spring functions to compensate for tolerances in the fitbetween the lens barrel and VLD block. This feature permits preciseadjustment of d while using inexpensive, easy to manufacture componentsin mass production applications. The lens barrel and lens together aremounted within the other end of the VLD block, as shown. Threads 77A areprovided on the exterior surface of the lens barrel, while matchingthreads 76A are provided on the interior surface of the bore 76Bextending through the VLD block 76. The pin hole 75A in the base of VLDyoke 75 pivots about pivot pin 73C on the pivot plate. This arrangementallows the position of the aspheric collimating lens to be adjustedrelative to the fixed position of the VLD, during a configurationprocedure to be described in great detail hereinbelow. A spring 81 isinserted into the end of bore 76B which produces a resistive forceagainst the lens barrel as it is threaded into the bore. When the VLDyoke, VLD, lens barrel and aspheric collimating lens are assembledtogether as a single adjustable subassembly, then the adjustable unit ispivotally supported in a gimbal like manner within the yoke by way ofsupport pins 78A and 78B, shown in FIG. 15G1 which pass through bores79A and 79B in yoke 75 and screw into thread holes BOA and BOB,respectively, in the VLD Block 76. This arrangement allows the directionof the laser beam from the lens barrel to be adjusted in the up and downdirection, relative to the face of the prism and thus the planar mirror.Also, the pivotal mounting of the yoke relative to the base plate,permits the orientation of the yoke, and thus the direction of the laserbeam, to be pivotally adjusted relative to the face of the prism duringthe configuration procedure. Additionally, the pivotal mounting of thepivotal base plate within the recess of the module optical bench allowsthe direction of the circularized beam emerging from the prism to beadjusted relative to the planar mirror. As will become apparent below,this adjustment mechanism permits the scanner designer to properlyconfigure the components of the VLD so that the above objectives aresatisfied in accordance with the principles of the present invention.

As shown in FIG. 16, there are three basic steps involved in the designof a laser beam production module according to the teachings of thepresent invention.

As indicated at Block A in FIG. 16, the first step of the module designmethod involves designing a first optical system comprising the i-thfacet and the fixed spatial frequency diffraction grating within thelaser beam production module. The sole function of this optical systemis to substantially eliminate laser beam dispersion during thediffraction of the incident laser beam through the rotating scanningdisc. In the first illustrative embodiment, the first optical systemcomprises fixed spatial-frequency diffraction grating (i.e. plate) 64and the i-th facet previously designed using the disc design method ofthe present invention. As indicated at Block B, the second step of themethod involves designing a second optical system comprising the VLD53A, aspheric collimating lens 61 and prism 62, configured so as tocircularize the laser beam produced from the VLD and eliminateastigmatism in the circularized beam beyond the prism. The third andfinal step is to couple the first and second optical systems by way ofplanar mirror 63 so as to form the laser beam production module of thefirst illustrative embodiment, shown in FIGS. 15A through 15K.Thereafter, the module can be parametrically configured and installedwithin the holographic laser scanner. The details of this process willbe described hereinbelow.

In FIG. 17A, the problem of beam dispersion during laser beamdiffraction through the scanning disc hereof is schematicallyillustrated. The parameters used to construct the geometrical opticsmodel of the beam diffraction process are shown in FIG. 17B. Therelationship between the grating parameters and the diffraction angleand the diffraction angle and the wavelength of the spectral componentsof the laser beam are defined in FIG. 17C. The graphical plot ofdiffraction angle versus wavelength shown in FIG. 17D explains why anincident laser beam produced from a conventional VLD tends to disperseas it is diffracted across the scanning facet. The various spectralcomponents associated with the VLD beam, due to superluminescence,multi-mode oscillation and mode hopping, exit the HOE facet on thescanning disc at different diffraction angles dependent on thewavelength thereof. The wavelength dependency of the diffraction angleis illustrated in 17D.

In order to minimize wavelength-dependent dispersion at each facet alongthe scanning disc of the present invention over the wavelength range ofconcern (e.g. 600 to 720 nanometers), the diffraction grating in thefirst optical system described above is positioned at a tilt angle ρ,defined as shown in FIG. 18A. The mathematical expression describing therelationship between the incidence angle θ_(i1), the diffraction angleθ_(dc1) at the reconstruction wavelength, the wavelength of the incidentbeam λ_(c) and the grating spacing d₁ is described by Expression No. 1in FIG. 18C. This equation is simply the grating equation whichdescribes the behavior of fixed frequency diffraction gratings, such asthe compensation plate 63 used in the first optical system. Then usingalgebraic techniques upon Expression No. 1, an expression for θ_(d1)(λ)can be derived as provided by Expression No. 3 in FIG. 18C. Therelationship between the angle of diffraction of the diffraction gratingθ_(dc1), the angle of incidence at the i-th facet θ_(i2) and the tiltangle ρ is described by Expression No. 2 in FIG. 18C. This mathematicalexpression is derived using a number of well known trigonometricrelations. While each HOE facet on the designed scanning disc has avariable frequency fringe structure in order to realize its focal lengthf_(i), the design procedure models each facet as if it were a fixedfrequency grating. This assumption can be made without introducingsignificant error in the design as the goal of the first optical systemis to minimize the beam dispersion through the HOE facets over the rangeof diffraction angles for which the facets have been previously designedto produce the prespecified scanning pattern. In the illustrativeembodiment, the range of diffraction angles is from about 26.6° to about47.5°, with the average diffraction angle being about 37 degrees. Thusthis average diffraction angle 37° will be selected as the diffractionangle used to design the first optical system. This diffraction angle isindicated by θ_(dc2) in the geometrical optics model, and describes theaverage direction in which the diffracted laser beam is directed towardsthe beam folding mirrors in the scanning system. Using the aboveassumption about each HOE facet on the scanning disc allows the facet inthe first optical system to be modelled by the well known diffractionequation, expressed in the form of Equation No. 3 set forth in FIG. 18C.

In order to complete the design of the first optical system, it isnecessary to find a set of values for the parameters representing thefirst optical system which results in minimizing the deviation of theaverage diffraction angle θ_(d2) over the range of spectral wavelengthsthat can be expected to be produced from a conventional VLD used toconstruct the designed laser beam production module. Ideally, thedeviation is zero over the wavelength range of interest; however, thisis not achievable in practice. Instead, this deviation is minimized overthe wavelength range of interest. Finding the set of parameters thatwill achieve this objective can be achieved by the following procedure.

Using Expression No. 3 in FIG. 18C, the system designer selects thevalue of incidence angle θ_(i1) required by scanner height and widthdimension constraints, and thereafter evaluates Expression No. 3 for therange of wavelength values λ of interest. During this evaluation step,an initial value for the tilt angle ρ is selected, whereas parametersθ_(i2), d₂ and λ_(R) indicated in Expression No. 3 are known orderivable from the previous disc design process. In particular, d₂ isarrived at by selecting the average fringe spacing among the numerousvariable frequency holographic facets realized on the previously designscanning disc. For a large number of different wavelength values withinthe λ range, the diffraction angle θ_(d2) is then calculated so that theθ_(d2)(λ) can be plotted as a function of wavelength λ, as shown in FIG.18D. From this plot, the deviation can be determined and if it is notacceptable, then the above process is repeated using a different tiltangle ρ until the plot θ_(d2)(λ) has an acceptable deviation over thewavelength range of interest. After several interactions, an acceptableparameter value for tilt angle ρ will be found. At this stage of thedesign process, the incidence angle parameter θ_(i1), the constructionangle of diffraction θ_(dc1) and the specified nominal reconstructionwavelength λ_(R) provide a set of parameters sufficient to construct thediffraction grating (i.e. wavelength compensation plate) while the tiltangle ρ is sufficient to mount the diffraction grating relative toincidence point r₀ on the scanning disc so that beam dispersion isminimized over the range of spectral wavelengths produced by the VLDduring beam scanning operations. A set of parameters found to minimizebeam dispersion over the output bandwidth of the VLD are set forth inFIG. 18B1. These parameters are based on the disc design parameters forthe scanning disc and scanning pattern of the illustrative embodiment.As shown in FIG. 18D, these parameters result in diffraction anglesθ_(d) ₂ which are substantially the same for various spectral componentswithin the output bandwidth of the VLD. In practical terms, this meansthat each of these spectral components in the incident circularizedlaser beam will be diffracted at substantially the same angle from thescanning disc, minimizing the dispersion of the diffracted scanningbeam.

Notably, the above-described method of designing the first opticalsystem of the laser beam production module provides the system designerwith two degrees of design freedom as either the incidence angle θ_(i1)or the tilt angle ρ can be used as a design variable while the other isused as a design constraint. This inventive feature allows the angle ofincidence θ_(i1) to be markedly different from angle of diffractionθ_(d) ₂, while nevertheless minimizing beam dispersion through thescanning disc over the spectral bandwidth of the laser beam producedfrom the VLD. The design method permits the incidence angle θ_(i1) to beany one of a large range of values which allows the constructed laserbeam production module to be physically mounted on the system opticalbench between the optical bench and the scanning disc, within the widthdimension constraints of the scanner housing. The design method alsopermits the tilt angle ρ to be any one of a large range of values whichprovides the designer great flexibility in mounting the laser beamproduction module relative to the scanning disc and parabolic lightcollecting mirror disposed therebeneath. These features of the presentinvention assist the system designer in designing and constructing aholographic laser scanner having a scanner housing volume which has beenminimized in relation to its specified scanning volume.

Having designed the diffraction grating employed in the first opticalsystem of the laser beam production module, it is appropriate to brieflyaddress the construction of the same. Typically, there will be a greatneed to mass manufacture laser beam production modules embodying“wavelength-compensation” diffraction gratings, of the type describedabove. Thus, holographic mastering techniques are ideally used. Whileany suitable mastering technique can be used, it will be necessary innearly all instances to holographically record the master diffractiongratings at a recording wavelength λ_(C) which is different than itsreconstruction wavelength λ_(R). The reason for this is generally wellknown: it is difficult to make holographic gratings with highfringe-contrast at the reconstruction wavelength λ_(R), which in theillustrative embodiment is about 670 nanometers. Instead, it is easierto record the gratings at a spectral wavelength at which high-contrastfringes can be realized and then playback at the wavelength of the VLDsin the scanner. Presently, the preferred recording medium for recordingdiffraction gratings with high-contrast fringes is Dichromated Gelatin(DCG)) which exhibits its greatest sensitivity near 480 nm. Thus, a bluelaser beam is required during recording. In order to record thediffraction grating at its construction wavelength, and then reconstructthe same at another wavelength, it is necessary to translate (i.e.convert) its complete set of construction parameters {θ_(i1),θ_(dc1)}expressed at the reconstruction wavelength λ_(R), into a completecorresponding set of parameters expressed at the specified constructionwavelength λ_(C). The process illustrated in FIG. 19A through 19E isvirtually identical to the process shown in FIGS. 28A1 to 28D and can beused to carry out the necessary parameter conversions. Details regardingthe process of 19A through 19E can be found by referring to thedescription of the process of FIGS. 28A1 to 28D detailed above.Thereafter, using the converted set of construction parameters, theholographic diffraction gratings can be made using the converted set ofconstruction parameters and the holographic recording systemschematically represented in FIG. 19F.

In the illustrative embodiment, the preferred recording medium for thediffraction grating of the laser beam production module is DCG havingits maximum light sensitivity in the blue spectral range, and thus thenecessary construction wavelength for exposing this recording medium canbe produced by an Argon gas laser with a peak spectral output centeredat about 488 nanometers. The set of construction parameters determinedusing the above-described conversion method can be used to physicallyconstruct a “master” diffraction grating at the second (construction)wavelength λ_(C), and then one or more grating “copies” can be made fromthe master diffraction grating for mass production of the laser beamproduction modules.

After completing the design of the first optical system of the laserbeam production module, the second step of the design method involvesdesigning the second optical system thereof. As mentioned above, thefunction of the second optical system comprising the VLD, asphericcollimating lens and the beam circularizing prism, is to circularize thelaser beam produced from the VLD and completely eliminate astigmatismalong the circularized beam from a point beyond the second surface ofthe beam expanding prism. In order to design such an optical system, thepresent invention teaches geometrically modelling the production of thelaser beam from a semiconductor VLD, while describing the phenomenon ofastigmatism inherently introduced along the produced laser beam. Thisnovel modelling technique will be described in detail below.

In FIG. 20, a geometrical model is provided for a semiconductor VLDwhich produces a laser beam having astigmatism inherently introducedalong the laser beam. In general, it is well known that the laser beamproduced from conventional VLDs has two different beam components,namely: a first beam component having a very narrow dimension which isparallel to the width dimension of the VLD junction (i.e. resonantcavity); and a second beam component having a very wide dimension whichis parallel to the height of the VLD junction. For purposes ofexposition, the first beam component shall be referred to as the “Pexternal wavefront” of the produced laser beam, whereas the second beamcomponent shall be referred to as the “S external wavefront” of theproduced laser beam. These designations S and P refer to conceptualcylindrical wavefronts which spread in a direction perpendicular (S) orparallel (P) to the LVD junction and are not to be confused with the Swave-polarization and the P wave-polarization directions of the incidentlaser beam at the scanning disk surface, defined hereinabove. Asillustrated in FIG. 20, the “S external wavefront” of the produced laserbeam is deemed to originate from an “effective S source” located withinthe volumetric extent of the VLD junction, whereas the “P externalwavefront” of the produced laser beam is deemed to originate from an“effective P source” located within the volumetric extent of the VLDjunction. Inasmuch as the “effective P source” is spatially separatedfrom the “effective S source” by some distance δ, referred to as the“astigmatic difference” inherent in each VLD and statistically varyingfrom VLD to VLD, the geometrical model predicts that the “S externalwavefront” will diverge at a rate different than the “P externalwavefront” along the produced laser beam and therefore the laser beamwill exhibit astigmatism in the well understood sense. According to thisgeometrical model, nearly all of the power of these external wavefrontcomponents reside in the Electric Field vector of these electromagneticwavefronts and the polarization thereof is parallel to the widthdimension of the VLD junction, which is commonly referred to as“transverse electric” polarization, or the TE mode of oscillation of theVLD. According to the model, the S point source produces a cylindricalwavefront whose center of curvature is located at the S source, whereasthe P point source produces a cylindrical wavefront whose center ofcurvature is located at the P source. Details concerning the physics ofVLDs can be found in “Heterostructure Lasers” Parts A and B by H. C.Casey, Jr. and M. B. Panish, Academic Press 1978. Notwithstanding thisfact of VLD physics, it is important to understand that the “effective Ssource” and the “effective P source” are constructions of thegeometrical model which have been developed for the purpose of designingthe second optical system of the laser beam production module of thepresent invention in accordance with the objects of the presentinvention. While there may be structural correspondence between the“effective S source” and junction geometry and between “effective Psource” and junction geometry, there is no need to specify suchcorrespondences herein for purposes of the present invention. What isimportant to practicing this aspect of the present invention is toemploy this novel geometrical model of the VLD in order to design thesecond optical system as will be described hereinbelow. The advantage indoing so will become apparent hereinafter.

The method of designing the second optical system proceeds by modellingthe VLD with the geometrical model of FIG. 20 in relation to ageometrical optics model of the aspheric collimating lens and the beamcircularizing prism, as shown in FIG. 20A. In FIGS. 20B1, 20B2 and 20B3,the geometrical optics model of the second system of the laser beamproduction module is shown in greater detail. In particular, thesefigure drawings graphically illustrate the geometrical and opticalparameters used to construct the geometrical optics model of the secondsystem, namely: the location of the effective sources of the S and Pwavefronts associated with the VLD; δ, the astigmatic difference of theVLD, defined as distance between the effective sources of the S and Pwavefronts; f₁, the focal length of the aspheric collimating lens; d,the distance between the focal point of the aspheric collimating lensand the S wavefront (i.e. Beam) source; D₁, the diameter of the Pwavefront (i.e. Beam) leaving the aspheric collimating lens; D₂, theexpanded diameter of the P wavefront leaving the prism; M, the beamexpansion factor characteristic of the beam expanding prism, defined asD₂/D₁; n, the refractive index of the prism material; θ_(Pi1), the angleof incidence of the lower portion of the converging P beam at the faceof the prism; θ_(Pi2), the angle of incidence of the upper portion ofthe converging P beam at the face of the prism; φ_(P1), the angle ofconvergence of the P beam leaving the aspheric collimating lens; φ_(S1),the angle of convergence of the S beam leaving the aspheric collimatinglens; φ_(P2), the angle of convergence of the P beam leaving the prism;φ_(S2)=φ_(S1), the angle of convergence of the S beam leaving the prism;L_(P1), the image distance for the P source imaged by the asphericcollimating lens; L_(P2), the image distance for the P source afterinserting the beam expanding prism; L_(S1), the image distance for the Ssource imaged by the aspheric collimating lens; L_(S2)=L_(S1), the imagedistance for the S source after inserting the beam expanding prism;θ_(Pr1), the angle of refraction of the lower portion of the convergingP beam in the prism; θ_(Pr2), the angle of refraction of the upperportion of the converging P beam in the prism; α, the prism apex anglewhich equals θ_(Pr1) by design for sake of convenience;θ_(Pi3)=θ_(Pr1)−θ_(Pr2)=α−θ_(Pr2), the angle of incidence of the upperportion of the converging P beam at the second surface of the prism; andθ_(Pr3)=φ_(P2) the angle of refraction of the upper portion of theconverging P beam leaving the second surface of the prism. Collectively,these parameters constitute the geometrical optics model of the secondoptical system. Notably, the distance between the first surface of theprism and the principal plane of the collimating lens need not beconsidered as a parameter to the model provided that the entirecross-sectional diameter of the beam is incident (i.e. falls) upon thefirst surface of the prism, which is a very easy assumption to satisfyin practice.

In FIG. 20C1, a set of assumed values are presented for variousparameters in the model which can remain fixed during the designprocess. In FIG. 20D, a set of equations are provided which defineparticular relationships between certain parameters in the geometricaloptics model of the second optical system. The MATHCAD tool availablewithin the HSD Workstation of the present invention can be used torealize the geometrical optics model of the second optical system. Asclearly illustrated, Expressions No. 1 to 13 in FIG. 20D lead to thederivation of expressions for L_(P2) and L_(S2), the distances of theimage of the P source and the image of the S source after being imagedthrough the aspheric collimating lens and the beam expanding prism. Fromthese functions, the curvature of the S cylindrical wavefront as itimmediately emerges from the second surface of the prism can be definedas 1/L_(S2), whereas the curvature of the P cylindrical wavefront as itimmediately emerges from the second surface of the prism can be definedas 1/L_(P2). Expressed in other words, the radius of curvature of the Scylindrical wavefront as it immediately leaves the second surface of theprism is given by 1/L_(S2), whereas the radius of curvature of the Pcylindrical wavefront as it immediately leaves the second surface of theprism is given by 1/L_(P2).

It is well known that each VLD having a non-zero astigmatic difference,defined herein as δ, will produce a laser beam which exhibits astigmaticproperties. However, it has been discovered that, for each non-zerovalue of δ and assumed values of incidence angles θ_(Pi1) and θ_(Pi2),there exists a realizable value of d, at which the S and P cylindricalwavefronts leaving the second surface of the prism have equal radii ofcurvature, as indicated in the plot shown in FIG. 20E. Under suchoptical conditions, both the S and P cylindrical wavefronts emergingfrom the second surface of the prism are converging along the outgoingoptical axis of the prism at the same rate (by virtue of their equalradii of curvature) and the resulting wavefront is spherical and free ofastigmatic aberrations associated with the non-zero inherent astigmaticdifference in the VLD. Through rigorous quantitative analysis, it hasalso been discovered that small changes in the angles of incidenceθ_(Pi1) and θ_(Pi2) have a significant effect in altering the radius ofcurvature of only one of the cylindrical wavefronts (P wavefront) whileminimally affecting the radius of curvature of the P wavefront. Notably,this condition exists because in the geometrical optics model of theVLD, the P-source resides further away from the principal plane of theaspheric collimating lens than does the S-source. Consequently, themathematical structure of the geometrical model for the second opticalsystem suggests that parameters d, θ_(Pi1) and θ_(Pi2) be selected as“adjustable parameters” used during the parameter adjustment procedurehereof so that the above-described optical conditions are satisfied andthe astigmatism eliminated.

In view of these discoveries, it will be helpful to briefly discuss theoptical function that each of the components performs in the secondoptical system when its parameters are configured in theastigmatism-elimination case described above. Firstly, as stated above,the S and P sources represented within the VLD produce cylindricalwavefronts emanating from the location of these S and P sources,respectively. The function of the aspheric collimating lens is to passthe S and P wavefronts, while changing the radius of curvature for bothof these wavefronts, as well as their apparent center of curvature.Notably, in the second optical system, both the S and P wavefronts areassumed to propagate on axis, and therefore off-axis aberrations will benegligible and thus need not be considered. The function of the prism isto significantly change the radius of curvature of only one of thesecylindrical wavefronts, while minimally changing the radius of curvatureof the other cylindrical wavefront. This significant degree of change inthe radius of curvature is a strong function of the angles of incidenceθ_(Pi1) and θ_(Pi2) measured with respect to the first surface of theprism. This functional relationship and the manner in which suchdependency is established among the various parameters in the Mathcadmodel, can be readily seen by carefully examining Expressions No. 2through 13 set forth in FIG. 20D. Most importantly, the above analysisreveals that the design method of the present invention provides thedesigner with two degrees of freedom when finding the set of parametersthat satisfies the optical condition illustrated in the plot shown inFIG. 20E. In particular, the scanner designer may select a given valuefor the prism incidence angles θ_(Pi1) and θ_(Pi2) (i.e. the prism tiltangle θ_(Prism-tilt)), and then find the parameter value for d (i.e.,the distance from the focal length of the collimating lens to the Ssource), which eliminates astigmatism at the second surface of theprism. Alternatively, the scanner designer may select a given value forthe distance d, (i.e., by setting the VLD-to-lens separation D to aninitial value) and then find the parameter values for the prism tiltangle θ_(Prism-tilt) which eliminates astigmatism at the second surfaceof the prism. This is a very important fact inasmuch as it will bedesirable in many applications to control the ellipticity (i.e. aspectratio) of the spherical converging wavefront produced from the secondsurface of the prism. With this degree of freedom available in thesecond optical system of the present invention, the ellipticity of thespherical wavefront from the second surface of the prism can be easilycontrolled by selecting the appropriate prism incidence angles θ_(Pi1)and θ_(Pi2). This feature of the present invention is of great value inmany scanning applications. In particular, when scanning dot-matrixcodes on poorly printed codes, it will be desirable to produce laserbeams having an aspect-ratio so that the beam height is greater than thevoids present between code elements (e.g. bars). The use of such laserbeams has the effect of averaging out such voids and thereby improvesthe first-pass read rate of such codes.

In any particular design application, the approach that will be usedwill depend on, for example, the physical constraints presented by theholographic scanner design. In order to find the distance d or prismtilt angle θ_(prism-tilt) at which the optical condition of FIG. 20E isachieved, two different parameter adjustment procedures have beendeveloped. As will be described in greater detail below, thesetechniques are based on the mathematical structure of the model used tofind the conditions at which astigmatism is eliminated while theelliptically shaped laser beam is circularized while passing through theprism of the second optical system of the laser beam production moduleof the first illustrative embodiment.

In practice, it is not feasible to empirically measure the astigmaticdifference δ for each VLD to be used in the construction of a laser beamproduction module. Consequently, it is not feasible to use Expressions14 and 15 in FIG. 20D to compute the distance d for selected values ofincidence angles θ_(Pi1) and θ_(Pi2) using the mathematical expressionsfor the S and P source image distances. Instead, the approach adopted bythe design method of the present invention is to exploit the two degreesof freedom in the geometrical model of the second optical system andprovide two different procedures which may be used to adjust (i.e.configure) the parameters of the system to eliminate astigmatism,circularize the laser beam, and optionally to adjust the focal point ofthe spherical converging wavefront (i.e. the resulting beam) emergingfrom the second surface of the prism. To avoid obfuscation of thepresent invention, these two techniques will be first described ingeneral terms, with explanation of how the various steps in theprocedures affect geometrical properties of the S and P cylindricalwavefronts, as well as the resulting spherical wavefront produced fromthe second surface of the prism. Thereafter, two illustrativeembodiments of these parameter adjustment techniques will be describedwith reference to the parameter adjustment system according to thepresent invention shown in FIG. 21A, which can be used to adjust thegeometrical and optical parameters of assembled laser beam productionmodules so that the various objects of the present invention areachieved therein.

In general, the function of the parameter adjustment system 85 of FIG.21A is to allow the prism tilt angle, θ_(Prism-Tilt) and distance d, tobe adjusted during the assembly/alignment procedure so that anastigmatism-free laser beam with a desired aspect-ratio is produced.Notably, by definition of parameter d in FIG. 20A, adjustment theretocan be achieved by simply adjusting the VLD-to-lens separation D. Asshown, the parameter adjustment system 85 comprises an optical bench 86upon which a pivot plate mounting fixture 87 is stationarily mounted.The function of the pivot plate mounting fixture is to mount during theparameter alignment procedure, pivot plate 72 carrying an opticalsubassembly comprising the VLD, the barrel, the lens mount, and theyoke. A beam scanning device 88, such as Model No. 1180-GP from Photon,Inc., is mounted on the optical bench of the parameter adjustment systemalong a first optical axis 89 which passes through test focusing lens90, the optical axis of the second surface of the prism when the prismplatform 73 with the prism thereon is mounted within second recess inthe pivot plate. Also, a beam detector (e.g. quadrant detector) 91 ismounted on the optical bench along an optical axis 92 which passesthrough the center of the first surface of the prism when the prismplatform with the prism thereon is mounted within the second recess inthe pivot plate.

As indicated at Block A in FIG. 21B, the first step of the firstgeneralized parameter adjustment technique involves selecting values forall parameters in the geometrical optics model for the second opticalsystem except for the distance d and the incidence angles θ_(Pi1) andθ_(Pi2) (i.e. prism tilt angle θ_(prism-tilt)), which are treated asvariables in the process. As indicated at Block B, the second stepinvolves selecting initial values for parameters d and θprism-tilt whichcan be accomplished by virtually any criteria (θ_(prism-tilt)). Then asindicated at Block C in FIG. 21B, the procedure involves setting theincidence angles θ_(Pi1) and θ_(Pi2) so that a desired beam ellipticity(i.e. aspect ratio) is achieved on the second surface of the prism. Inthe event that a circular beam cross-section is desired at the secondsurface of the prism, and as well along the scanning beam, the aspectratio will be unity, whereas when an elliptical beam cross-section isdesired the aspect ratio will be some value not equal to unity. Inessence, this step presents a parameter constraint which the secondsystem must satisfy.

As indicated at Block D in FIG. 21B, the VLD-to-lens separation D isadjusted so as to find the parameter value d at which the radius ofcurvature of both the S and P cylindrical wavefronts are made equal atthe second surface of the prism, resulting in a spherical wavefrontthereat which is converging along the optical axis of the second opticalsystem. Under such conditions, the astigmatic difference between the Sand P wavefronts is completely eliminated at and beyond the secondsurface of the prism. However, in some instances the rate at which thespherical wavefront of the laser beam converges is so great that thefocal power of one or more of the holographic facets, working inconjunction with the focal power of the incident laser beam is so greatthat the beam focuses at a point in the scanning field short of orbeyond its prespecified focal plane. In order to compensate for thisexcessive or insufficient focal power, the disc designer may perform anadditional stage of parameter adjustment to increase or decrease theradius of curvature of the resulting spherical wavefront so that whenthe spherical wavefront of the laser beam passes through eachholographic facet on the scanning disc, the radius of curvature of thespherical wavefront will cause the wavefront to converge at theprespecified focal plane of the scanning pattern.

As indicated at Block E in FIG. 21B, the first step of this optionaladjustment stage involves varying d to adjust the radius of curvature ofthe S cylindrical wavefront not significantly affected by variations inthe prism tilt angle, and thereby cause both cylindrical wavefronts tofocus to a focal point which will ensure that the beam focuses onto thefocal plane of concern within the scanning volume of the holographicscanner. As indicated at Block F in FIG. 21B, the second step of theoptional adjustment procedure involves adjusting the prism tilt angleuntil the radius of curvature of the cylindrical wavefront sensitive toprism tilt θ_(Prism-tilt) (i.e. the S wavefront) is once again equal tothe radius of curvature of the other cylindrical wavefront, thusproducing a spherical wavefront at the second surface of the prism whichis converging along its optical axis. Inasmuch as this readjustment stepseeks to achieve a desired focal length (i.e. the imaging distance) andeliminate astigmatic difference along the spherical wavefront of thelaser beam, it is not possible to guarantee a circularized beam. Inshort, it is only possible to control precisely either ellipticity orfocal length of the laser beam while eliminating astigmatism, but notboth, in the second optical system.

As indicated at Block A in FIG. 21C, the first step of the secondgeneralized parameter adjustment technique involves realizing values forall parameters in the geometrical optics model the second optical systemexcept for the distance d and the incidence angles θ_(Pi1) and θ_(Pi2),which are treated as variables in the process. As indicated at Block Bin FIG. 21C, the second step involves selecting an initial value forparameters d and θ_(pi1) and θ_(Pi2) θ_(prism-tilt) which can beaccomplished by virtually any criteria. Then as indicated at Block C inFIG. 21C, the procedure involves setting the distance d so that the Scylindrical wavefront, which is not sensitive to variations in prismtilt angle, is focused to a desired focal length, which may or may notbe necessary to compensate for the focal power of the holographicfacets, in relation to the prespecified focal planes in the scanningvolume. In essence, this step at Block C presents a parameter constraintwhich the second system must satisfy. As indicated at Block D in FIG.21C, the prism tilt angle is then adjusted so that the radius ofcurvature of the S cylindrical wavefront, which is not sensitive toprism tilt angle adjustment, is made equal to the radius of curvature ofthe P cylindrical wavefront, which is sensitive to prism tilt angleadjustment at the second surface of the prism, resulting in a sphericalwavefront thereat which is converging along the optical axis of thesecond optical system. Under such conditions, the astigmatic differencebetween the S and P wavefronts is completely eliminated at and beyondthe second surface of the prism. As the beam diameter (or aspect ratio)at the second surface of the prism is substantially equal to the beamdiameter (i.e. aspect ratio) at the scanning disc, there is no need toreadjust this parameter using a parameter readjustment stage of the typeprovided in the first parameter adjustment procedure.

When the design of the first and second optical systems of the laserbeam production module have been completed, the next step of the processis to couple these systems. This step is achieved using plane mirror 63which receives the astigmatism-free beam from the second surface of theprism and directs it through the diffractive grating at the predesignedincidence angle, at which it diffracts and ultimately falls incident onthe rotating scanning disc. In essence, the plane mirror simply changesthe direction of the laser beam from the prism and couples it to thediffraction grating without modifying the beam cross-section or otherproperties of the laser beam. In the illustrative embodiment, the planemirror functions to fold the laser beam so that the aspheric collimatinglens, prism and grating can be arranged in a manner to realize necessaryparameters, while minimizing the volume within which the laser beamproduction module is realized. While a plane mirror has been used tocouple the first and second optical systems together, it is understoodthat in other embodiments of the present invention, these systems can bedirectly coupled by proximate positioning, without the interposition ofan optical component therebetween.

It is appropriate at this juncture to describe a specific procedure forassembling the components of the laser beam production module of thefirst illustrative embodiment, and configuring the geometrical andoptical parameters thereof in accordance with the principles of thepresent invention. This particular procedure is based on the secondgeneralized parameter adjustment method described above using theoptical bench shown in FIG. 21A. As indicated in Blocks A, B and C ofFIG. 21C1, the few steps of the procedure involve assembling theabove-described subassembly upon the pivot plate. Specifically, the VLD53A (53B, 53C) is first press fitted into one end of the VLD block 76.Then the aspheric collimating lens 61 is mounted in one end of the lensbarrel 77. Then the lens barrel is screw-mounted into the VLD block byturning the same 3-4 turns. This step carries out the initial setting ofthe parameter d. As indicated at Block D, the VLD/lens subassembly isthen attached to the VLD yoke 75 by way of pins 78A and 78B pivotallysupporting the VLD and lens subassembly with one degree of rotationalmovement relative to the VLD yoke. Thereafter, at Block E the VLD yoke75 is rotatably mounted to pivot plate 72 by way of pivot axis 73C, asshown in FIG. 21A. At Block F of FIG. 21C1, the pivot plate and opticalsubassembly mounted thereon is placed within fixture plates 87 of theparameter adjustment bench.

Without the prism not yet mounted to the pivot plate, the next stage ofthe procedure is carried out in order that the laser beam produced fromthe VLD and aspheric lens assembly is directed along an axis which willintersect the prism when mounted on the pivot plate and ensure that itsentire beam cross-section falls incident upon the first surface of theprism. This stage of the procedure is carried out using the beamphotodetector 91 mounted along axis 92, shown in FIG. 21A. The firststep of this stage indicated at Block H involves tilting the VLD/lenssubassembly within the yoke so that the laser beam is directed alongtarget axis 92 and falls upon the quadrant-type photodetector. Ifnecessary, one may adjust the beam size on the target by rotating thelens housing barrel 77 within the VLD block 76, and thus adjust theVLD-to-lens separation D. At Block I of FIG. 21C2, the yoke assembly isthen rotated until the laser beam passes through the cross-hair of thetarget at the beam photodetector 91. So configured, the VLD and lenssubassembly and yoke assembly are both locked in the position whichensures that the laser beam crosses through the target cross-hair, andthus the first surface of the prism.

The next stage of the procedure indicated at Block J of FIG. 21C2involves installing the prism support plate 73 (with the prismpremounted thereon) within the second mounting recess within the pivotplate, so that an initial value of prism tilt angle, θ_(prism tilt) isset. Then at Block K of FIG. 21C2, the lens barrel is adjusted relativeto the VLD block, setting d so that the cross-sectional dimension of thebeam in the non-scanning direction (i.e. parallel to code elements—barsand spaces) focuses to the focal length of the test lens 90 in FIG. 21A.

At this stage, an optical subassembly is provided having all of theessential components for configuring θ_(prism tilt) sufficient toeliminate astigmatism while achieving a desired beam aspect ratio.

Then Block L of FIG. 21C2 involves adjusting the prism tilt angleθ_(prism tilt) so that astigmatism is eliminated while achieving aparticular beam aspect ratio. This stage involves the use of the Photon®Beam Scanning device to measure the beam cross-section of the laser beamin x and y directions, at different points along the optical axis of theprism, along which the beam propagates away from the second surfacethereof. This prism tilt angle adjustment step is carried out selectinga prism tilt angle, and then measuring the beam cross-section along thebeam. When the cross-sectional measures of the beam indicate that thebeam converges to its focal point at the same rate along the x and ydirections, and then diverges at equal rates in these orthogonaldirections as one moves the point of measurement along the length of thebeam, then the value of prism tilt angle, denoted θ*_(prism-tilt), atwhich such conditions are detected is the prism tilt angle at whichastigmatism is completely eliminated along the laser beam. Onceobtained, this parameter θ*_(prism-tilt) is locked into position usingan adjustment screw and/or adhesive, as indicated at Block M of FIG.21C2.

When the laser beam production module has been completely assembled andits parameters configured to eliminate astigmatism, the pivot plate isthen mounted within the recess of the optical bench of the laser beamproduction module and then the pivot plate is rotated relative to modulebench until the beam is perpendicular to the mirror, as indicated atBlock N of FIG. 21C2. This step involves using the quadrant detectorset-up along a different test optical axis. Then at Block O of FIG.21C3, the light diffractive grating and mirror can be mounted within thesupport of the optical bench. Then at Block P of FIG. 21C3, the entirelaser beam production module can be mounted on the optical bench of thescanning system, using alignment pins and holes, as illustrated in FIG.21D. At this stage, the laser beam emanating from the second surface ofthe prism is automatically oriented along an axis which ultimatelypasses through the scanner disc in the plane formed between the (i) lineextending from the outer scanning disc to Beam-Incident-Point pointr_(o) and (ii) the scanning disc axis of rotation itself. At this stageof the construction process, angle of incidence θ_(i2) is automaticallyset so that the laser beam dispersion is minimized as the laser beam isdiffracted through the scanning disc. This is achieved by physicalconstruction of the scanner bench and module bench supporting thegrating. Notably, angle of incidence θ_(i2) has been previouslydetermined by the design process for the first optical system. Once theoutgoing laser beam from the laser beam production module is alignedwith respect to the scanning disc, the optical bench of the module canbe fixed in place using bolts, screws or other fasteners known in theart. The above procedure is repeated for each of the other two laserbeam production modules.

Laser Beam Production Module of the Second Illustrative Embodiment

In FIG. 22, an alternative embodiment of the laser beam productionmodule of the present invention is shown. In this second embodiment ofthe module, the use of a prism is eliminated. Instead, only a VLD 53A,an aspheric colliminating lens 61, a planar mirror 63 and adual-function light diffraction grating 95 of fixed spatial-frequencyare used to construct the module, as shown in FIG. 23. As shown in FIG.23, all other components of the scanning system are the same.

In FIG. 23A, the components of the laser beam production module 12A′(12B′, 12C′) of the second illustrative embodiment are shown assembledon the optical bench of the module, removed from the scanner housing.The construction of this embodiment of the laser beam production moduleis similar in many respects to the first illustrative embodiment, inthat it has a pivot plate 72′ upon which a VLD yoke 75 is pivotallymounted for pivotally supporting VLD yolk Block 75. The VLD 53A andaspheric lens 65 are mounted with the lens barrel 77 as described aboveand this subassembly in turn is pivotally mounted within the VLD yoke75. In this embodiment, there is a planar mirror 63 stationarily mountedwith respect to module bench 60 by support elements. Also, thedual-function light diffraction grating 64′ is stationarily mounted withrespect to the planar mirror. In order to adjust the angle of incidenceat which the laser beam reflected off the planar mirror strikes thedual-function light diffraction grating, pivot plate 72′ is pivotallyadjustable with respect to the optical bench of the laser beamproduction module in a manner to that provided in the first illustrativeembodiment. Using this optical assembly, the laser beam productionmodule assembly, the laser beam production module can be realized,achieving the above-described objects of invention.

As shown in FIG. 24, the method for designing the laser beam productionmodule of the second illustrative embodiment of the present inventioninvolves three basic steps. As indicated at Block A in FIG. 24, thefirst step involves designing a first optical system which includes thedual-function light diffractive grating 64′ and the holographic facetson the predesigned scanning disc. The first optical system has twoprincipal functions, namely: to control the aspect-ratio of the incidentlaser beam on the scanning disc; and to minimize laser beam dispersionover the bandwidth of the VLD as the laser beam is diffracted throughthe rotating scanning disc. As indicated at Block B of FIG. 24, thesecond step of the design process involves designing a second opticalsystem using the previously designed dual-function light diffractivegrating. The principal function of the second optical system is toeliminate astigmatism along the laser beam at the second optical surfaceof the diffractive grating. At Block C of FIG. 24, the design processinvolves coupling the first and second optical systems using the planarmirror 63 to form a single unitary module which, when coupled with thescanning disc, performs the three above-described optical functions in ahighly reliable manner. Each of these steps will be described in greaterdetail hereinafter.

Referring to FIGS. 25A through 25E and FIG. 26, the design of the firstoptical system of the laser beam production module of FIG. 23 will bedescribed in detail.

As shown in FIG. 25A, a geometrical optics model is constructed for thefirst optical system, based on two assumptions, namely: (1) that theradius of curvature of the spherical wavefront incident the facet isvery large relative to the surface area of the facet; and (2) that alllight rays thereof are substantially collimated (i.e. the incidentwavefront is substantially planar over the facet surface area). As shownin FIG. 25A and defined in the parameter description table of FIG. 25B,this model includes a number of external angles and distances, namely:D₁, the diameter of the laser beam leaving the aspheric collimatinglens; D₂, the expanded diameter of the laser beam after emerging fromthe second surface of the dual-function diffractive grating; M, the beamdiameter expansion ratio, defined as D₂/D₁; d₂, the average gratingspacing of the facets on the scanning disc (in microns); θ_(i2), theincidence angle defined relative to a normal vector drawn to the firstsurface of an exemplary holographic facet on the scanning disc;diffraction angle θ_(d2), defined relative to a normal vector drawn tothe second surface of the holographic facet; incidence angle θ_(i1),defined relative to a normal vector drawn to the first surface of thedual-function light diffraction grating; diffraction angle θ_(d1)defined relative to a normal vector drawn to the second surface of thelight diffraction grating; θ_(i1M), the angle of incidence of the beamat the dual-function light diffraction grating that will provide thedesired beam expansion ratio, M; θ_(i1D), the angle of incidence of thebeam at the dual-function light diffraction grating that will providezero dispersion for the beam leaving the scanning disk; θ_(d1M), theangle of diffraction of the beam leaving the dual-function lightdiffraction grating that will provide the desired beam expansion ratio,M; θ_(d1D), the angle of diffraction of the beam leaving thedual-function light diffraction grating that will provide zerodispersion for the beam leaving the holographic disc; the orientation(i.e. tilt) angle ρ defined between the holographic disc and themulti-function light diffraction grating; and λ, the wavelength of thelaser beam (in microns) produced from the VLD.

In FIG. 25C, a set of mathematical expressions are provided which definerelationships between the parameters of the geometrical optics model.Expression No. 1 in FIG. 25C, derived from the well known gratingequation, relates d₂, the “average grating spacing” of the fringestructures in the scanning disc, to the reconstruction wavelength of theVLD λ, and incidence angle θ_(i2) and diffraction angle θ_(d2). Usingtrigonometric relations, the angle of diffraction θ_(d1M) at which thedesired beam expansion ratio occurs can be defined in terms of the tiltangle ρ and incidence angle θ_(i2) as defined by Expression No. 3 inFIG. 25C. Starting with the well known beam expansion ratio equationM=Cos(θ_(d1))/Cos(θ_(i1)), and applying some algebraic manipulation andEquation No. 3, a mathematical expression for the incidence angleθ_(i1M) can be derived in terms of tilt angle ρ, which will have theform of Expression No. 2 in FIG. 25C. Then using the grating equation,the grating spacing for the dual-function light diffraction grating canbe derived as a function of the tilt angle ρ and the incidence anddiffraction angles θ_(i1M) and θ_(d1M), respectively. This mathematicalexpression is set forth as Expression No. 4 in FIG. 25C.

Then using trigonometric relations, the angle of diffraction θ_(d1D) atwhich zero beam dispersion occurs is defined in terms of the tilt angleρ and incidence angle θ_(i2) as defined by Expression No. 6 in FIG. 25C.Starting then with a zero dispersion equation similar to Expression No.3 in FIG. 18C, and applying Expression No. 6 and some algebraicmanipulation, an expression for the incidence angle θ_(i1D) is derivedin terms of tilt angle ρ, incidence angle θ_(i2), reconstructionwavelength λ_(R) of the VLD, and d₂ (the average grating spacing of afixed-spatial frequency equivalent of the holographic facets). This formof this expression is described by Expression No. 5 in FIG. 25C. Thenusing the grating equation once again, the grating spacing d_(1M)(ρ)associated with the dual-function light diffractive grating, is derivedas a function of the tilt angle ρ, the incidence angle θ_(i1D) andwavelength λ_(R).

Thereafter, assuming values for parameters λ_(R),θ_(i2), M and θ_(d) ₂,as set forth in the table of FIG. 25B1, Expressions 3 and 5 in FIG. 25Ccan be expressed solely as a function of tilt angle ρ. Notably,diffraction angle θ_(d2) is selected to be the average of the variousdiffraction angles (e.g. 37 degrees) associated with the sixteenholographic facets on the designed scanning disc of the illustrativeembodiment. The beam expansion factor M, on the other hand, willtypically be selected as the ratio of the two beam spread angles for theVLD used in the laser beam production module (e.g. M=3.0). However, inorder to ease the manufacturing of the dual-function grating, the beamexpansion factor M may be chosen somewhat smaller than the ratio ofthese beam spread angles. In the illustrative embodiment, the wavelengthof the laser is 0.670 microns, whereas the angle of incidence at thescanning disk is 43 degrees while the corresponding angle of diffractionis 37 degrees, the average value thereof falling near the middle of therange of diffraction values for the 16 holographic facets on thescanning disk.

In order to find the value of tilt angle ρ at which both the conditionsexpressed in Expressions 2 and 5 are simultaneously satisfied, one oftwo solving techniques may be used. The first technique involvesequating Expressions 2 and 5 in FIG. 25C equal to each other and thensolving for tilt angle ρ. Alternatively, the second technique involvesplotting the functions expressed in Expressions 2 and 5, as a functionof tilt angle ρ, and identifying the value of tilt angle ρ₀ at whichthese functions intersect. Notably, by setting the tilt angle ρ betweenthe scanning disk and the dual-function diffraction grating equal to ρ₀,the first optical system will achieve a beam expansion ratio of M=3.0while minimizing beam dispersion over the bandwidth of the VLD producingthe incident beam. In the illustrative embodiment, the value of tiltangle ρ₀ is equal to −11.1 degrees, at which the angle of incidenceyielding the desired beam expansion ratio also equals the angle ofincidence that yields minimum beam dispersion over the bandwidth of theincident laser beam produced by the VLD.

In FIG. 25E, a set of construction parameters are provided for thedual-function grating of the illustrative embodiment. Notably, theseparameters are expressed at the reconstruction wavelength 670nanometers, and thus must be converted to the construction wavelength ofthe Argon laser when the specified light diffraction grating is to berealized in DCG. The parameter conversion system and procedure of FIGS.28A1 through 28D described above can be used for this purpose.

A post design tool available within the HSD workstation hereof, referredto as the “Beam Dispersion Analyzer” tool, is illustrated in FIGS. 27Athrough 27D1. This analytical tool can be used to analyze the variationsof diffraction angle θ_(d2) for the laser beam leaving a designedscanning disc, geometrical modelled in FIG. 26. This tool is of greatvalue in measuring the degree to which beam dispersion has been reducedwhen using a multi-function diffraction grating and any particular setof construction parameters (including parameters for tilt angle ρ)specified by the above-described design process.

As shown in FIG. 26, the second optical system designed above isgeometrically modelled in a manner similar to that done during thedesign process. Parameters used to construct the geometric optics modelare described in FIG. 27A. Given (assumed) parameters are set forth inFIG. 27B for the illustrative embodiment. Mathematical expressionsdescribing important relations among certain of the parameters are setforth in FIG. 27C. In Expression No. 4 in FIG. 27C, the angle ofdiffraction θ_(d2) is expressed as a function of wavelength (in air) λ,tilt angle ρ, grating spacing d₁, grating spacing d₂, and incidenceangle θ_(i1). Assuming parameter values for ρ, d₁, d₂, and θ_(i1),Expression No. 4 can be reduced to a function dependent solely onwavelength. Then by evaluating this resulting function using differentvalues of wavelength within the bandwidth of the VLD, a plot ofdiffraction angle θ_(d2) can be plotted, as shown in FIGS. 27D and 27D1,a measure of beam dispersion derived. Notably, the laser bandwidth orspread from commercially available VLDs will be about 0.010 microns orless, and thus this will be a sufficient domain for λ. Typically,wavelength variations due to mode hopping are on the order of 0.0003microns. With such assumed wavelength shifts from the VLDs in thescanning system, the resulting plot from the Beam Dispersion Analyzerindicates that first optical system of the module designed above willmaintain the angular deviation (i.e. beam dispersion) of its diffractedlaser beam to about 0.0055 degrees.

After completing the design of the first optical system of the laserbeam production module, the dual-function light diffraction grating usedtherein can be constructed using holographic recording techniques. Usingthe grating equation, this fixed spatial-frequency light diffractivegrating (HOE) can be uniquely specified by its reconstruction wavelengthλ_(R) and the angle of incidence θ_(i1) and angle of diffraction θ_(i1)required by the design. However, as explained in connection with thedesign of the scanning disc and the laser beam production module of thefirst illustrative embodiment, it is easier to construct (i.e.fabricate) the dual-function diffraction grating at a constructionwavelength λ_(C) different than reconstruction wavelength λ_(R),selected on the basis of the recording emulsion (e.g. DCG) used torealize the dual-function grating. The parameter conversion processillustrated in FIGS. 28A1 through 28D can be used to convertconstruction parameters for the dual-function grating, into acorresponding set of construction parameters expressed at theconstruction wavelength λ_(C). When calculating the exposure angles atthe construction wavelength, the Bragg plane angle within the emulsionmust be maintained constant after the construction process. As thisprocess has been described in connection with the construction of eachholographic facet on the scanning disc of the present invention, thedetails thereof will not be repeated herein to avoid redundancy. Afterthe parameter conversion process of FIGS. 28A1 through 28D is carriedout, the dual-function diffraction grating can be fabricated using thewavelength-converted parameters and the recording system illustrated inFIG. 29.

The next step in the design process entails designing the second opticalsystem for the laser beam production module. In FIG. 30A, a geometricaloptics model of the second optical system is shown. In the secondillustrative embodiment, the sole function of this optical system is toeliminate astigmatism from the system. Consequently, the constraintsimposed on this system design will differ from those applied in thefirst illustrative embodiment. As illustrated, the geometrical opticsmodel comprises the VLD, the aspheric collimating lens, and thedual-function diffraction grating designed above as a fixedspatial-frequency holographic diffraction grating. Various geometricaland optical parameters of the geometrical optics model are indicated inFIGS. 30A, 30A1 and 30A2 and defined in detail in the table ofparameters set forth in FIG. 30B. As described in FIG. 30B, thegeometrical optics model of the second optical system is formed by thefollowing parameters: f₁, the focal length of the aspheric collimatinglens; S-source, the location of the source of the S cylindricalwavefront (i.e. S-beam source); P-source, the location of the source ofthe P cylindrical wavefront (i.e. P-beam source); d, the distancemeasured from the focal point of the collimating lens to the location ofthe source of the S cylindrical wavefront (i.e. S-beam source); δ, thedistance between the S-source and the P-source (i.e. the astigmaticdifference); D₁, the diameter of the P wavefront leaving the asphericcollimating lens; D₂, the diameter of the expanded P wavefront leavingthe dual-function light diffraction grating; M, the beam expansionfactor, defined as M=D₂/D₁; d_(h), the grating spacing of thedual-function light diffraction grating, measured in microns; θ_(Pi1),the angle of incidence of the lower portion of the converging Pwavefront at the front surface of the dual-function light diffractivegrating; θ_(Pi2), the angle of incidence of the upper portion of theconverging P wavefront at the front surface of the dual-function lightdiffractive grating; φ_(P1), the angle of convergence of the P wavefrontleaving the second surface of the aspheric collimating lens; φ_(S1), theangle of convergence of the S wavefront leaving the second surface ofthe aspheric collimating lens; φ_(P2), the angle of convergence of the Pwavefront leaving the second surface of the dual-function lightdiffractive grating; φ_(S2), the angle of convergence of the S wavefrontleaving the second surface of the dual-function light diffractivegrating, which is equal to φ_(S1); L_(P1), the image distance for the Pwavefront imaged by the aspheric collimating lens; L_(P2), the imagedistance for the P wavefront imaged by the aspheric collimating lensafter insertion of the dual-function light diffractive grating; L_(S1),the image distance for the S wavefront imaged by the asphericcollimating lens; L_(S2), the image distance for the S wavefront imagedby the aspheric collimating lens, after inserting the dual-functionlight diffractive grating, which is equal to L_(S1); θ_(Pd1), the angleof diffraction of the lower portion of the converging P wavefront at thedual-function light diffractive element; θ_(Pd2), the angle ofdiffraction of the upper portion of the converging P wavefront at thedual-function light diffractive element; and λ_(r), the reconstructionwavelength of the laser beam produced from the VLD. Collectively, theseparameters constitute the geometrical optics model of the second opticalsystem of the second illustrative embodiment of the laser beamproduction module. Notably, the distance between the first surface ofthe dual-function holographic light diffractive grating and theprincipal plane of the collimating lens need not be considered as aparameter to the model provided that the entire cross-sectional diameterof the beam is incident (i.e. falls) upon the first surface of the lightdiffractive grating, which is a very easy assumption to satisfy inpractice.

In FIG. 30B1, a set of assumed values are presented for variousparameters in the model which can remain fixed during the designprocess, providing various coefficients in the mathematical expressionswithin the geometrical optics model. In FIGS. 30C1 and 30C2, a set ofmathematical expressions are provided which define particularrelationships between certain parameters in the geometrical optics modelof the second optical system. As clearly illustrated, Expressions No. 1to 12 lead to the derivation of equations for L_(P2) and L_(S2), givenby Expressions Nos. 11 and 12 in FIGS. 30C1 and 30C2, the imagedistances of the P source and the S source after being imaged throughthe aspheric collimating lens and the light diffractive grating. Fromthese functions, the curvature of the S cylindrical wavefront as itimmediately emerges from the second surface of the light diffractivegrating can be defined as 1/L_(S2), whereas the curvature of the Pcylindrical wavefront as it immediately emerges from the second surfaceof the light diffractive grating can be defined as 1/L_(P2). Expressedin other words, the radius of curvature of the S cylindrical wavefrontas it immediately leaves the second surface of the light diffractivegrating is given by L_(S2), whereas the radius of curvature of the Pcylindrical wavefront as it immediately leaves the second surface of thelight diffractive grating is given by L_(P2). Mathcad 3.1 mathematicaldesign program can be used to carry out geometrical optics modellingwithin the HSD Workstation of the present invention.

It is well known that each VLD having a non-zero astigmatic difference,defined herein as δ, will produce a laser beam which exhibits astigmaticproperties. However, it has been discovered that, for each non-zerovalue of δ and assumed value of grating tilt angle θ_(grating-tilt)(i.e. grating incidence angles θ_(Pi1) and θ_(Pi2)), there exists arealizable value of d, at which the S and P cylindrical wavefrontsleaving the second surface of the light diffractive grating have equalradii of curvature, as indicated in the plot shown in FIG. 30D. Undersuch optical conditions, both the S and P cylindrical wavefrontsemerging from the second surface of the light diffractive grating areconverging along the outgoing optical axis of the light diffractivegrating at the same rate (by virtue of their equal radii of curvature)and the resulting wavefront is spherical and free of astigmaticaberrations associated with the non-zero inherent astigmatic differencein the VLD. The mathematical structure of the geometrical model for thesecond optical system suggests that, during the parameter adjustmentprocedure hereof, the geometrical parameter d functions as a variable or“adjustable parameter” while the grating tilt angle θ_(grating-tilt)parameter and θ_(Pi2) determined hereinabove function as constraints sothat the optical conditions for astigmatism-elimination can be foundduring the adjustment procedure.

The optical functions performed by each of the components in the secondoptical system of this embodiment are similar to the functions performedby the components in the second optical system of the first illustrativeembodiment. In particular, the S and P sources represented within theVLD produce cylindrical wavefronts emanating from the S and P sourcelocations, respectively. The optical function of the asphericcollimating lens is to pass the S and P wavefronts, while changing theradii of curvature for both of these wavefronts as well as theirapparent centers of curvature. In this embodiment of the second opticalsystem, both the S and P wavefronts are assumed to propagate on axis,and therefore off-axis aberrations will be negligible and thus need notbe considered. The optical function of the light diffractive grating inthe second optical system is to significantly change the radius ofcurvature of only one of these cylindrical wavefronts, while minimallychanging the radius of curvature of the other cylindrical wavefront.This significant degree of change in the radius of curvature is a strongfunction of the angles of incidence θ_(Pi1) and θ_(Pi2) measured withrespect to the first surface of the light diffractive grating. Thisfunctional relationship and the manner in which such dependency isestablished among the various parameters in the analytical model of thisoptical system can be readily seen by carefully examining Expressions 1through 12 set forth in FIGS. 30C1 and 30C2.

Importantly, the above analysis reveals that the design method of thesecond illustrative embodiment provides the designer with two degrees offreedom when finding the set of parameters that satisfies the opticalcondition illustrated in the plot shown in FIG. 30D. In particular, thedesigner may select a given value for the grating incidence anglesθ_(Pi1) and θ_(Pi2), and then find the parameter value for parameters dwhich eliminates astigmatism at the second surface of the lightdiffractive grating. Alternatively, the designer may select a givenvalue for the distance d, and then find the parameter value for gratingtilt angle θ_(grating-tilt) which eliminates astigmatism at the secondsurface of the light. Notably, in this illustrative embodiment, theamount of tilt angle adjustment is quite small (e.g. 2-3 degreesmaximum) due to the inherent Bragg angle sensitivity of thedual-function light diffractive grating of the laser beam productionmodule.

Notably, the mathematical structure of the second optical systemdescribed above allows either (i) for the distance d to function as asystem constraint in the parameter adjustment procedure, while thegrating tilt angle θ_(grating-tilt) functions as the variable parametertherein, or (ii) for the grating tilt angle θ_(grating-tilt) to functionas a system constraint in the parameter adjustment procedure, while thedistance d functions as the variable parameter therein. Based on thesetwo facts, two different parameter adjustment procedures have beendeveloped to find the distance d or grating tilt angle θ_(grating-tilt)which eliminates astigmatism. While these techniques are based on themathematical structure of the model used to find the conditions at whichastigmatism is eliminated, they are not limited to laser scanningsystems. In any particular design application, the procedure that willbe used to configure the parameters of the optical system comprising aVLD, an aspheric collimating lens and a light diffractive grating, willdepend on the physical constraints presented by the application at hand.For example, in designing the laser beam production module of the secondillustrative embodiment where the grating tilt angle is predeterminedwhen designing the first optical system thereof, the grating tilt angleθ_(grating-tilt) functions as a constraint during the design of thesecond optical system, whereas the distance parameter d functions as thevariable parameter. In the case where a laser beam production module isbeing designed for a non-holographic laser scanner, and thus gratingtilt angle θ_(grating-tilt) is not constrained to any particular value,then this parameter may function as a variable in the geometrical opticsmodel of the optical system.

As mentioned in connection with the design of the laser beam productionmodule of the first illustrative embodiment, it simply is not feasiblein practice to empirically measure the astigmatic difference δ for eachVLD to be used in the construction of a laser beam production module ofthe second illustrative embodiment. Consequently, it is not feasible touse the mathematical expressions set forth in the table of FIGS. 30C1and 30C2 to compute the distance d for selected parameter values.Instead, the approach adopted by the design method of the secondillustrative embodiment is to exploit the structure of the geometricalmodule described above and provide a novel procedure and bench foradjusting (i.e. configuring) the parameters of the second optical systemto eliminate astigmatism. For clarity of exposition, the parameteradjustment bench will be described first , and thereafter, a generalizedversion of the parameter adjustment procedure with reference to theprocess diagram of FIG. 31B. Finally, a particular illustrativeembodiment of the procedure will be described with reference to theparameter adjustment bench of FIG. 31A and process diagram of FIG. 31C.

In FIG. 31A, a parameter adjustment system 100 of the present inventionis shown for use with the above-described laser beam production module.The function of this bench is to allow the parameters grating tilt angleθ_(grating-tilt) and distance d to be adjusted during theassembly/alignment procedure so that an astigmatism-free laser beam witha desired aspect-ratio is produced. As illustrated in FIGS. 31A1 and31A2, the parameter adjustment system comprises an optical bench 101upon which a pivot plate mounting fixture 102 is stationarily mounted.The function of the pivot plate mounting fixture is to mount during theparameter alignment procedure, an optical subassembly comprising modulebench 60′ and pivot plate 72′ with the VLD, barrel, lens mount, and VLDyoke assembled thereon. The pivot plate mounting fixture provides apivot plate mounting recess designed to securely receive the modulebench 60′ and its associated optical subassembly.

As shown in FIG. 31A1 and 31A2, the parameter adjustment systemcomprises beam scanning device 88 mounted on the optical bench along afirst optical axis which, when the light diffractive grating 72′ ismounted on grating platform 70, passes through the center of the secondsurface of the light diffractive grating 72′ along an optical axis 103passing through a scanning disc emulation hologram (H2) 104, a test lens(having length f_(test)) 105, and x-y beam scanner 88, as shown in FIG.31A2. This adjustment mechanism allows the laser beam to be prealignedrelative to the second surface of the light diffractive grating, withoutthe light diffractive grating being mounted during the alignment step.The reason that scanning-disc emulation hologram 104 is required isbecause the dual-function diffraction grating, by itself, introducesdispersion which would affect the measurements without the use of afixed frequency grating 104 which corresponds to an “average”holographic facet, with no focal power (e.g. θ_(i)=−43°, θ_(d)=37°).Notably, hologram 104 is tilted at angle ρ with respect to thedual-function grating to give zero beam dispersion. If the incidenceangle θ (i.e. θ_(grating-tilt)) is changed during the design of thefirst optical system, then ρ preferably should be changed in order toimprove the reduction of beam dispersion.

As shown in FIGS. 31A1 and 31A2, the parameter alignment bench alsocomprises a beam detector (e.g. quadrant-type photodetector) 91. Thebeam detector 91 is mounted on the optical bench along a second opticalaxis 106 which, when the light diffractive grating is mounted on gratingplatform 70 of module bench 60′, passes through the center of the firstsurface of the light diffractive grating 72′. As will be describedbelow, these test instruments are used to adjust the geometrical andoptical parameters of the laser beam production module during theassembly and configuration of the laser beam production module.

Generalized parameter adjustment technique, analogous to the generalizedmethod described in FIG. 21B, will now be described with reference toFIG. 31B. Notably, this generalized technique is preferred inasmuch asit permits the dual-function grating to be fixedly mounted to the modulebench, and thus predesigning the module bench and scanner bench so thatautomatic configuration of ρ is set upon mounting the module bench tothe scanner bench via alignment pins 67 and 68. As indicated at Block Athereof, the first step of the technique involves realizing values forall parameters in the geometrical optics model of the second opticalsystem except for (i) the distance d which is treated as a variable inthe process and (ii) the grating angle θ_(grating-tilt) which is treatedas a constraint in the design process. As indicated at Block B of FIG.31B, the second step involves setting parameter θ_(grating-tilt) whichis obtained from the design process of the first optical system of thelaser beam production module of the second illustrative embodiment. Uponsetting this parameter, the specified aspect ratio should be obtained.If the specified aspect ratio is not obtained upon setting parameterθ_(grating-tilt) to the value determined in the design process of thefirst optical system, the grating tilt angle should be adjusted untilthe desired aspect ratio is obtained. Then as indicated at Block C ofFIG. 31B, the distance d is adjusted so that the radius of curvature ofboth the S and P cylindrical wavefronts are made equal at the secondsurface of the light diffractive grating, resulting in a sphericalwavefront thereat which is converging along the optical axis of thesecond optical system. Under such conditions, the astigmatic differencebetween the S and P cylindrical wavefronts is completely eliminated atand beyond the second surface of the dual-function light diffractivegrating.

Upon completing the design of the constituent optical systems of thelaser beam production module of the second illustrative embodiment, thenext step of the process, indicated at Block D of FIG. 31B, involvescoupling together the first and second optical systems to provide alaser beam production module mounted on the scanner bench withprecomputed incidence angle θ_(pi1) (i.e. θ_(grating-tilt)) andprecomputed grating tilt angle ρ set, minimizing laser beam dispersionover the bandwidth of the VLD. In this illustrative embodiment, thefirst and second optical systems of the laser beam production module aredirectly coupled without the use of an intermediate optical element,such as a planar mirror. However, in an alternative embodiment, a planemirror may be used to fold the laser beam between the asphericcollimating lens and the light diffractive grating. This system couplingtechnique may be desirable in particular applications, where theaspheric collimating lens, and grating must be arranged in a mannerrelative to the scanning disc to realize a laser beam production modulewith miniature volumetric dimensions which must satisfy particularphysical constraints.

It is appropriate at this juncture to describe a particular procedurefor assembling the components of the laser beam production module of thesecond illustrative embodiment, and configuring the geometrical andoptical parameters thereof in accordance with the principles of thepresent invention.

As indicated at Blocks A, B, C and D of FIG. 31C1, the first stage ofthe particular procedure involves assembling the above-describedsubassembly upon the pivot plate. Specifically, at Block A, the VLD isfirst press-fitted into one end of the VLD block 76. At Block B theaspheric collimating lens 61 is mounted in one end of the lens barrel77. At Block C, the lens barrel is then screw mounted into the VLD blockby turning the same 3-4 turns or so to set the distance parameter d tosome initial value. At Block D, the VLD/lens subassembly is thenattached to the VLD yoke 75 by way of pins 78A and 78B to pivotallysupport the VLD and lens subassembly with one degree of rotationalmovement relative to the VLD yoke. Thereafter, at Block E of FIG. 31C1,the VLD yoke is rotatably mounted to pivot plate 72′ shown in FIG. 23A.At Block F, the pivot plate and optical subassembly mounted thereon isthen mounted on module bench 60′. At Block G, module bench 60′ with itssubassembly shown in FIG. 23A, is then placed within the recess of themounting fixture 102 of the parameter adjustment bench of FIGS. 31A1 and31A2. At this stage of the assembly/adjustment procedure, indicated atBlock H, electrical power is applied to the VLD so that it produceslaser beam output.

The next stage of the procedure uses the beam photodetector 91 of theparameter adjustment system to align the produced laser beam with thefirst optical axis of the light diffraction grating. Without thedual-function light diffractive grating mounted to include bench 60′ andwith the parameter adjustment bench arranged as shown in FIG. 31A1, thefirst step of this stage, indicated at Block I of FIG. 31C1, involvestilting the VLD/lens subassembly within the yoke so that the laser beamis directed along target axis 106 (i.e. to the first optical axis of thelight diffractive grating) and falls upon the target (i.e. quadrant-typephotodetector 91). At Block J at FIG. 31C2, the VLD yoke assembly isthen rotated until the laser beam passes through the cross-hair of thetarget at the beam photodetector 91. Notably, the target position isselected so that when the grating and mirror are installed the laserbeam strikes the mirror at a position which reflects the beam on Braggthrough the dual function grating, as well as on an optical axis whichis coplanar with the axis of rotation of the holographic scanning disc.When so configured, the VLD and lens subassembly and yoke assembly areboth locked in the position.

The next stage of the procedure indicated at Block K of FIG. 31C2involves installing the mirror 63 and dual function grating 72′ inmodule bench 60′ as shown in FIG. 31A2, using any suitable adhesive orequivalent means. With the diffraction grating and mirror mounted to themodule bench, an optical subassembly is now provided having all of theessential components for configuring parameter d sufficient to eliminateastigmatism while achieving predetermined beam aspect-ratio.

As indicated at Block L of FIG. 31C2, the next stage of the procedureinvolves adjusting parameter d by rotating lens barrel relative to VLDblock so that astigmatism is eliminated. This stage is carried out usingthe Photon® Beam Scanning device 88, volume hologram (H2) 103, and testlens 105 arranged in the manner illustrated in FIG. 31A2. While the VLDis actively driven and a laser beam emanating from the second surface ofthe light diffractive grating, parameter d is incrementally adjusted byrotating the lens barrel relative to the VLD Block C until astigmatismis eliminated. During this incremental adjustment process, the Photon®Beam Scanning device is used to measure the beam cross-section of thelaser beam in x and y directions, at different points along the opticalaxis of the grating and colliminating lens along which the beampropagates. Specifically, this adjustment step is carried out byselecting a value for d, and then measuring the beam cross-section alongthe beam. When the cross-sectional measures of the beam indicate thatthe beam converges to its focal point at the same rate along the x and ydirections, and then diverges at equal rates in these orthogonaldirections as one moves the point of measurement along the length of thebeam, then the value of distance d, denoted d*, at which such conditionsare detected, is the value of d at which astigmatism is completelyeliminated along the laser beam. When this parameter value for d isfound by the above adjustment procedure, astigmatism is eliminated atthe second surface of the light diffractive grating and therebeyond.This value of parameter d* can then be locked with adhesive or othersuitable means.

When the laser beam production module has been completely assembled andits parameters configured to eliminate astigmatism, then, as indicatedat Block M in FIG. 31C2, the entire laser beam production module,preconfigured in the manner described above, is mounted on the opticalbench of the scanning system as illustrated in FIG. 31D, so thatalignment pins 68 on module bench 60′ fit into alignment holes 69 inscanner bench 5. At this stage of the assembly process, the grating tiltangle ρ₀ is automatically configured (i.e. set) so that laser beamdispersion is minimized as the laser beam is diffracted through thescanning disc. Notably, grating tilt angle ρ₀, previously determined bythe design process for the first optical system, is set by thepredesigned angle at which grating 72 is mounted on module bench 60′,relative to the geometry of scanner bench 5 and module bench 60′. Oncethe laser beam production module is aligned as described above, it isthen fixed in place using bolts, screws or other fasteners known in theart. The above described procedure is repeated for each laser beamproduction module at each scanning station within the holographic laserscanner.

Design of the Light Collecting and Detecting Subsystem of the PresentInvention

Having described in great detail various procedures for designing andmaking holographic scanning discs and laser beam production modulesaccording to the present invention, it is appropriate at this junctureto describe various light collection/detection subsystems for use in theholographic laser scanner of the present invention, and methods ofdesigning the same.

As shown in FIGS. 14 and 22, the laser scanning system of theillustrative embodiment employs a light collecting/detecting subsystemwhich comprises three major subcomponents, namely: the holographic facetof the scanning disc 7 used to produce the P(i,j)th scanning plane fromwhich reflected laser light being collected originated; a paraboliclight focusing element (e.g. a parabolic focusing mirror) 14A (14B, 14C)mounted beneath the scanning disc adjacent each laser scanning station,and a photodetector 15A (15B, 15C) mounted above the scanning disc,along the focal axis of the parabolic light focusing mirror. Asmentioned hereinabove, this subsystem design allows the scanner designerto minimize the height dimension of the scanner housing beneath thescanning disc, while the height of the beam folding mirrors determinesthe height of the scanner housing above the scanning disc.

The constraints which must be satisfied by an acceptable design for thelight collection/detection subsystem of the present invention arespecified as follows: (1) substantially all of the reflected light rayscollected by any particular holographic facet during a light collectionoperation and focused by the parabolic light focusing mirror, passthrough the particular holographic facet at an angle in which the lightdiffraction efficiency is minimal in order that maximal optical power istransmitted through the holographic facet towards the photodetectorlocated at the focal point of the parabolic light focusing mirror; (2)the light rays reflected from a scanned code symbol falling incident onthe inner and outer (i.e. extreme) portions of the holographic scanningdisc during light collection operations (i.e. indicated as R₁ and R₂ inFIG. 34) are strongly diffracted by the scanning disc in a directionanti-parallel to the angle of incidence of the outgoing laser beam uponthe scanning disc during laser beam scanning operations; and (3) thesurface area of the parabolic focusing mirror is of such spatial extentand arranged relative to the scanning disc and photodetector thatsubstantially all light rays collected by a particular holographic facetduring a light scanning operation are received by the parabolic lightfocusing mirror as the holographic scanning disc rotates about its axiswithin the holographic laser scanner of the present invention. Theseconstraints are important to the design and operation of the lightcollection subsystem shown in FIGS. 14 and 22, and as such, are embodiedwithin the steps of the method for designing the light collectionsubsystem of the present invention described below. While possible,analytical expressions could be formulated for the geometrical opticsmodel of the subsystem as shown in FIG. 32, and thereafter, optimaldesign parameters obtained through a rigorous mathematical analysis, aswas employed with regard to the other subsystems of the holographicscanner hereof. However, the approach adopted below is to use theabove-described subsystem constraints to provide a procedure fordesigning a suitable light collection subsystem for use with thepreviously designed scanning disc and laser beam production module ofthe present invention.

As indicted at Block A in FIG. 33A, the first step of the design methodinvolves light diffraction efficiency analysis (i.e. Bragg sensitivityanalysis) for each holographic facet in the previously designed scanningdisc. The goal of this analysis is to determine, in the outgoingdirection of the scanning disc, the angle of incidence relative to theBragg angle of the facet (i.e. off Bragg), at which the lightdiffraction efficiency of the facets drops below a predetermined minimalthreshold. Alternatively stated, the goal is to determine the angularrange of incidence angles (e.g. from θ_(A) to θ_(B)) outside of whichthe diffraction efficiency of the holographic facet drops below thepredetermined minimal threshold. This angular range is schematicallyillustrated in the geometrical model FIG. 34. As will be describedbelow, this information is theoretically derived from an analysis of thediffraction efficiency of the facets with respect to particularpolarization states of the light focused by the parabolic mirror. Themathematical expressions used to analyze such light diffractionefficiency as a function of incidence angle θ_(i) will differ for thedifferent illustrative embodiments of the scanning disc hereof. Ingeneral, three types of holographic scanning disc may be used in anyparticular scanner design, namely: a scanning disc designed for usewithout cross-polarizers before the photodetectors; a scanning disc foruse with P polarizers before the photodetectors; and a scanning disc foruse with S polarizer before the scanning disc, as described above. Thus,Bragg sensitivity analysis for each of these three cases will bedescribed below. In each case, a precise 3-D geometrical model of theholographic laser scanner under design is created, using the parametervalues for the various subcomponents thereof determined in prior stagesof the scanner design process hereof. Preferably, the 3-D geometricalmodel produced at this stage should not represent the parabolic lightfocusing mirrors 14A, 14B, 14C, nor the photodetectors 15A, 15B, 15C, asthe precise geometry and relative position of the parabolic mirrors havenot been specified at this stage of the design process, nor have theprecise locations of the photodetectors been specified. The partialnature of the geometrical model is illustrated in FIG. 34. As willbecome apparent hereinafter, several critical design stages, involvinglight diffraction efficiency and ray tracing analysis, must first beperformed before such specifications can be accurately obtained inaccordance with the principles of the present invention.

As indicated at Block B of FIG. 33A, the next stage of the designprocess involves using the HSD workstation to perform a BraggSensitivity Analysis on each facet of the holographic scanning disc todetermine the range of incident angles off Bragg, at which light raysreflected off the parabolic mirror will be transmitted through thefacets with minimal diffraction towards the photodetector. Thegeometrical optics model shown in FIGS. 35A and 35B is used to representthe relevant geometrical and optical parameters used in the constructionof a Bragg Light Diffraction Sensitivity Model of the holographicfacets, based upon the original theoretical foundations laid down inKogelnik's paper, supra. As the geometrical optics model of FIG. 35A1 isvirtually identical to the model described in FIGS. 10A2 through 10B, itwill not be necessary to repeat here the description of the geometricaland optical parameters comprising this model.

In FIGS. 35B1 and 35B2, a Bragg Light Diffraction Sensitivity Model isprovided for the scanning disc designed for use without cross-polarizersbefore the photodetectors, shown in FIG. 10A1. This model contemplatesthat light of both S and P polarization states is reflected from ascanned code symbol, collected by the holographic facet, focused by theparabolic mirror and eventually transmitted through the holographicfacet onto the photodetector for detection. Consequently, Expression No.14 in FIG. 35C2 provides an expression for the “average” diffractionefficiency for light of S and P polarization states transmitted througheach particular facet on the scanning disc, as a function of the angulardeviation from the Bragg angle δ_(e). The constituent S and Pdiffraction efficiencies described by Expressions 12 and 13 of FIG.35C2, respectively, are formulated using the assumed parameter valueslisted in the table of FIG. 35B1. The mathematical expressions set forthin Expressions No. 1 through 11 in FIG. 35C1 are derived by applicationof Snell's Law to the geometrical optics model of FIGS. 35A1 and 35A2,and principles of the Coupled Wave Theory in volume-type holographiclight diffraction gratings, described in great detail in HerwigKogelnik's paper, supra. Notably, while the “obliquity factors” C_(S)and C_(R) defined in Equations 6 and 7 are expressed in terms of theinternal incidence angle α and the fringe slant angle φ, theseparameters can be expressed in terms of θi and θd, as discussed inKogelnik's paper.

The functions plotted in FIGS. 35D1 and 35D2 show the “normalized”average light diffraction efficiency for the first and sixteenthholographic facet, expressed as a function of the angular deviation fromthe Bragg angle, δ_(e). Expression No. 14 is used to produce suchgraphical plots. For δ_(e)=0, which is the case where the angle ofincidence is equal to the Bragg angle of the holographic facet, thetheoretical average light diffraction efficiency is maximum (i.e.E_(norm./avg.)=1) as one would expect. For angles of incidence away fromthe Bragg angle of the facet, the light diffraction efficiency generallydecreases, with some oscillatory behavior. By evaluating and plottingthe “normalized” average light diffraction efficiency for eachholographic facet, the subsystem designer can identify, for eachholographic facet, at which angle off Bragg δ_(e) the normalized lightdiffraction efficiency is below a minimal threshold (e.g. 0.09). Usingsuch angular information, the designer can determine at which anglesfocused light rays from the parabolic mirror must be transmitted throughthe holographic facets with minimal diffraction, and thus maximum powertransfer for detection. Notably, it has been found during such analysisthat in order to reflect the collected light rays back through thescanning disk toward the photodetector without significant diffractionlosses, the angle of incidence of each and every one of the light raysfrom the complete bundle of light rays from the parabolic mirror, mustbe at least 20 degrees away from the outgoing beam angle of incidence(i.e. the outgoing Bragg angle).

Referring to FIGS. 37A through 37C2, and the geometrical optics model ofthe scanning disc shown in FIGS. 28A1 and 28A2, a Bragg LightDiffraction Sensitivity Model will be described for analyzing thescanning disc designed with an S polarizer placed before thephotodetectors, as shown in FIG. 36 i.e., when using the laser beamproduction module of the second illustrative embodiment. This modelcontemplates that light of P polarization state is used to scan a codesymbol, and light of S polarization state is reflected from a scannedcode symbol, collected by the holographic facet, focused by theparabolic mirror and eventually transmitted through the holographicfacet onto the photodetector for detection. The S polarizer allows lightrays of S polarization to pass onto the photodetector, whereas lightrays of P polarization state are filtered out by the polarizer.Consequently, Expression No. 12 in FIG. 37B provides a generalexpression for the diffraction efficiency of each particular facet onthe scanning disc to light of S polarization state transmittedtherethrough. Notably, this characteristic of each facet is expressed asa function of the angular deviation from the Bragg angle be and has beenformulated using the assumed parameter values listed in the table ofFIG. 37A1. The mathematical expressions set forth in Expressions No. 1through 11 are derived by application of Snell's Law to the geometricaloptics model of the volume-type holographic facets on the scanning disc,as shown in FIGS. 35B1 and 35B2. The “obliquity factors” C_(S) and C_(R)defined in Expressions No. 6 and 7 of FIG. 37B are derived using thewell known principles of the Coupled Wave Theory in volume-typeholographic gratings. The functions plotted in FIGS. 37C1 and 37C2 showthe “normalized” light diffraction efficiency for the holographic facetsNo. 1 and 16 to S polarized light, expressed as a function of theangular deviation from the Bragg angle, δ_(e). Expression No. 12 in FIG.37B is used to produce such graphical plots. For δ_(e)=0, which is thecase where the angle of incidence is equal to the Bragg angle of theholographic facet, the theoretical light diffraction efficiency of eachfacet to S polarized light is maximum (i.e. E_(norm.)=1) as one wouldexpect. For angles of incidence away from the Bragg angle of the facet,the light diffraction efficiency generally decreases, with someoscillatory behavior. By evaluating and plotting the “normalized” lightdiffraction efficiency for each holographic facet, the subsystemdesigner can identify, for each holographic facet, at which angle offBragg δ_(e) the normalized light diffraction efficiency is below aminimal threshold (e.g. 0.09). By analyzing such plots, the designer canthen determine at which angles focused light rays from the parabolicmirror must be transmitted through the holographic facets with minimaldiffraction, and thus maximum power transfer for detection.

Referring to FIGS. 38A through and the geometrical optics model of thescanning disc shown in FIGS. 28A1 and 28A2, a Bragg Light DiffractionSensitivity Model is provided for the scanning disc designed for usewith a P state polarizer placed before the photodetectors, as shown inFIG. 36 i.e., when using the laser beam production module of the firstillustrative embodiment hereof. This model contemplates that light of Spolarization state is used to scan a code symbol, and light of Ppolarization state is reflected from a scanned code symbol, collected bythe holographic facet, focused by the parabolic mirror and eventuallytransmitted through the holographic facet onto the photodetector fordetection. The P polarizer allows light rays of P polarization state topass onto the photodetector, whereas light rays of S polarization stateare filtered out by the polarizer. Consequently, Expression No. 12 inFIG. 38B2 provides a general expression for the diffraction efficiencyof each particular facet on the scanning disc to light of P polarizationstate transmitted therethrough. Notably, this characteristic of eachfacet is expressed as a function of the angular deviation from the Braggangle δ_(e) and has been formulated using the assumed parameter valueslisted in the table of FIG. 38A1. The mathematical expressions set forthin Expression Nos. 1 through 11 of FIG. 38B1 are derived by applicationof Snell's Law to the geometrical optics model of the volume-typeholographic facets on the scanning disc, as shown in FIGS. 35A1 and35A2. The “obliquity factors” C_(S) and C_(R) defined in Expressions 6and 7 of FIG. 38B1 are derived using the well known principles of theCoupled Wave Theory in volume-type holographic gratings. The functionsplotted in FIGS. 38C1 through 38C2 show the “normalized” lightdiffraction efficiency for holographic facet Nos. 1 and 16 to Ppolarized light, expressed as a function of the angular deviation fromthe Bragg angle, δ_(e). Expression No. 12 is used to produce such afamily of graphical plots. For δ_(e)=0, which is the case where theangle of incidence is equal to the Bragg angle of the holographic facet,the theoretical light diffraction efficiency of each facet to Ppolarized light is maximum (i.e. E_(norm.)=1) as one would expect. Forangles of incidence away from the Bragg angle of the facet, the lightdiffraction efficiency generally decreases, with some oscillatorybehavior. By evaluating and plotting the “normalized” light diffractionefficiency for each holographic facet, the subsystem designer canidentify, for each holographic facet, at which angle off Bragg δ_(e) thenormalized light diffraction efficiency is below a minimal threshold(e.g. 0.09). By analyzing such plots, the designer can then determine atwhich angles focused light rays from the parabolic mirror must betransmitted through the holographic facets with minimal diffraction, andthus maximum power transfer for detection.

Having completed the Bragg Sensitivity Analysis required for the type ofscanning disc employed in the scanner under design, the subsystemdesigner can then locate the position (e.g. center and optical axisorientation) of the photodetectors above the scanning disc. As indicatedat Block C in FIG. 33A, this step involves using the HSD workstation toconduct an accurate ray training analysis of all incoming light raysreflected from a code symbol anywhere in the scanning volume onto thefacets of the scanning disc, and based on this analysis, identifying apoint above the scanning disc (but below the top edge of the beamfolding mirrors) which is free of incoming light rays. At Block D, usethe ray free points to locate the position of the photodetectors.

As indicated at Block E in FIG. 33A, the next step in the subsystemdesign method involves selecting a generalized parabolic surfacefunction S_(parabolic)(x,y,z) for use in specifying the lightcollection/focusing mirror of each light collection subsystem. As willbe described below, the balance of the subsystem design method theninvolves specifying the parameters of the parabolic surface patch, fromwhich the parabolic mirror can be constructed.

As indicated at step F in FIG. 33B, the next step of the subsystemdesign process involves extending the geometrical optics model of thesubsystem by adding to the geometrical optics model of FIG. 34, a linewhich extends from the center location of the photodetector, parallel toand preferably above the line of laser beam incidence to the scanningdisc, as shown in FIG. 39. The function of this line is to establish theposition and orientation of the optical axis of the yet, unspecifiedparabolic surface path representative of the parabolic mirror to beconstructed and installed beneath the scanning disc, adjacent the laserbeam production module.

As indicated at Block G of FIG. 33B, the next step of the design methodinvolves specifying the focal length of the parabolic surface patch. Thefocal length of the parabolic surface patch will typically be determinedprimarily by spatial restrictions beneath the scanning disc. In theholographic laser scanner of the illustrative embodiment, the focallength for the parabolic surface was chosen to be 3.0 inches as thisprovided sufficient space below the scanning disc to mount the parabolicmirror. It is understood, however, this parameter will typically varyfrom embodiment to embodiment.

As indicated at Block H in FIG. 33B, the next step of the design methodinvolves determining which holographic facet on the scanning disc hasthe smallest inner radius, r_(i). By its very geometry, this facet willcollect light rays closest to the center (i.e. hub) of the scanningdisc, and thus will diffract light rays closest to the axis of rotationthereof. Thereafter, use this facet to determine the lengthwisedimension of the parabolic surface patch, as shown in FIG. 39. Notably,for purposes of design, the extreme (i.e. inner and outer) light raysfalling on this facet are assumed to strike the surface thereof at theBragg angle of the facet, and thus by design, the diffracted light raysare transmitted towards the parabolic surface patch in a directionparallel to the optical axis of the parabolic surface patch. In thisway, when the parabolic mirror is realized according to thespecification of the parabolic surface patch, collected light raysfalling incident on a facet close to the Bragg Angle thereof will befocused to the focal point of the parabolic light focusing surface, atwhich the photodetector is located.

As indicated at Block I of FIG. 33C, the next step of the design methodinvolves determining which holographic facet on the scanning disc hasthe greatest angular rotation, θ_(rot). As will be described below, thisfacet will be used to specify the widthwise dimension of the parabolicsurface patch. The lower bound set on the widthwise dimension of theparabolic surface patch is the design constraint requiring thatsubstantially all light rays collected by a particular holographic facetduring a light scanning operation are received by the parabolic mirroras the holographic scanning disc rotates about its axis of rotation. Theupper bound on the widthwise dimension of the parabolic surface patch isthe available space beneath the scanning disc, within thespatially-constrained housing.

At Block J of FIG. 33C, the subsystem designer uses the 3-D geometricaloptics model of the scanner developed heretofore on the HSD workstationand the facet with the greatest angular sweep to determine the minimalleft and right surface boundaries that may be imposed upon the widthwisedimensions of the parabolic surface patch. Below is a technique fordetermining these surface boundaries.

As shown in FIG. 40A, the minimal left surface boundary is determined bycomputer modelling in 3-D, the situation where the incident laser beamhas just begun to illuminate the rightmost edge of the above-identifiedholographic facet. Ideally, at this stage of the scanline generationprocess, all of the reflected light rays reflected off the beam foldingmirror are collected by the holographic facet. However, to ensure thatall such light rays collected by the facet at this stage of the scanningoperation are collected by the parabolic light focusing mirror forfocusing, the designer extends outwardly the leftmost surface boundaryof the parabolic surface patch just so that the entire facet is disposedbeneath the parabolic surface patch.

Then as shown in FIG. 40B, the minimal right surface boundary isdetermined by computer modelling in 3-D, the situation where theincident laser beam is just about finished illuminating the leftmostedge of the above-identified holographic facet. Ideally, at this stageof the scanline generation process as well as at other instancesthereof, all of the reflected light rays reflected off the beam foldingmirror are collected by the holographic facet. To ensure that all suchlight rays collected by the facet at this stage of the scanningoperation are collected by the parabolic light focusing mirror forfocusing, the designer extends outwardly only the rightmost surfaceboundary of the parabolic surface patch just so that the entire facet isdisposed beneath the parabolic surface patch.

Having completed the above steps, the widthwise surface dimensions canbe determined by projecting the boundaries determined at the scanningdisc plane, onto the 3-D parabolic surface patch. Collectively, thelengthwise and projected widthwise dimensions of the parabolic surfacepatch provide “patch cutting parameters” that can be used to construct aparabolic mirror for the light collection subsystem under design. Apreferred way of constructing the parabolic mirror is to use the patchcutting parameters to cut out a parabolic patch from a parabolic mirrorhaving the focal distance specified at Block G of the design procedure.Notably, the resulting parabolic mirror designed above, will cover theentire area of the light collecting portion of the largest facet overthe entire sweep width as the scanning disc rotates.

Then at Block K of FIG. 33C, the 3-D geometrical model of the lightcollection/detection subsystem is revised on the HSD workstation usingthe complete set of specifications for the parabolic surface patch (i.e.parabolic mirror). Then as indicated at Block L of FIG. 33C, the updatedgeometrical model is carefully analyzed on the HSD workstation toconfirm that all light rays reflected off the parabolic mirror aretransmitted through the respective holographic facets off Bragg, toensure that maximum optical power is transmitted to the photodetector atthe focal point of the parabolic mirror of the light detectionsubsystem. If this ray tracing analysis proves that the subsystem designsatisfies the specified criteria, then the design process is completedand the subsystem design can then be realized according to the finalgeometrical model. If, however, the ray tracing analysis indicates thedesign falls short of satisfying its criteria, the designer can returnto any one or more of the above-described steps in the procedure, modifythe parameters thereat, and proceed through the design process until thedesired performance criteria is satisfied. Typically, one run throughthis design procedure is all that will be required to achieve asatisfactory subsystem design which satisfies the system constraintspresented at this stage of the overall scanner design process.

In FIG. 41, the holographic laser scanner hereof is shown with analternative embodiment of the light detection subsystem of the presentinvention. Instead of using a parabolic mirror to focus collected lightrays towards a photodetector located at the focal point of the parabolicmirror, this scanning system employs a reflection-volume hologram 108 toperform such an optical function. In all other respects, the lightdetection subsystem of FIG. 41 is similar to the illustrative embodimentdescribed in detail hereinabove. Notably, the design techniquesdescribed above can be used to design the reflection-volume hologram 108of the light detection subsystem. Using the complete specifications forthe parabolic surface patch, from which the parabolic mirror wasdesigned and constructed, the reflection-volume hologram can beconstructed in a manner which will now be readily apparent in view ofthe disclosure hereof.

As shown in FIGS. 42 through 43B, two alternative embodiments of theholographic laser scanner of the present invention are shown. Theseholographic scanning systems are similar to the illustrative embodimentsdescribed hereinabove, except for the structure of the light detectionsubsystems employed therein.

The light detection subsystem of the embodiment shown in FIG. 42comprises photodetector 15A and a system of light collection andfocusing optics 110 which avoids folding light rays collected andfocused beneath the scanning disc. The light collecting and focusingoptics comprise a planar light collecting mirror 111 and acondenser-type focusing lens 112. As shown, the light collecting mirror111 is disposed beneath the outer portion of the scanning disc, forreceiving parallel light rays falling incident upon and collected by theholographic facet at its Bragg angle. The parallel light rays collectedby the planar mirror are directed substantially parallel to the plane ofthe scanning disc, and are focused by focusing lens 112 to its focalpoint at which the photodetector 15A is located. One disadvantage ofusing this light detection subsystem design is that it requires agreater volume of space beneath the scanning disc to accommodate mirror111, and focusing lens 112 which will typically require a relativelyshort focal length, at which the photodetector is placed. From apractical point of view, this can often require the placement of thescanning disc motor above, rather than below, the scanning disc, asshown in FIG. 42.

The light detection subsystem of the embodiment shown in FIGS. 43A and43B comprises a photodetector 15A and a system of light collection andfocusing optics 113 which folds and focuses collected light rays beneaththe scanning disc. The light collecting and focusing optics comprise afirst planar ray-folding mirror 114, a second planar ray-folding mirror115, and a condenser-type focusing lens 116. As shown, the planar lightcollecting mirror is disposed beneath the outer portion of the scanningdisc, for receiving parallel light rays falling incident upon andcollected by the holographic facet at its Bragg angle. The parallellight rays collected by the planar mirror 114 are directed substantiallyparallel to the plane of the scanning disc onto folding mirror 115. Theray folding mirror 115 in turn redirects the collected light raystowards focusing lens 116, located under the scanning disc. The focusinglens 15A focuses the folded light rays to its focal point at which thephotodetector 15A is located. As shown, each photodetector is realizedon the analog signal processing board of the associated scanningstation. As with the above-described embodiment, a major disadvantage ofusing this light detection subsystem design is that it requires muchspace beneath the scanning disc, often requiring the placement of thescanning disc motor above, rather than below, the scanning disc asshown.

While the holographic scanner of the present invention and its numerousmethods of system design have been described in great detail withreference to the use of volume-transmission holograms, it is understoodthat volume-reflective holograms can be used to construct theholographic scanning disc of the present invention employed in thevarious embodiments of the holographic scanning system hereof. In FIG.44, such an alternative embodiment of the scanning system of the presentinvention is shown constructed using a scanning disc realized from aplurality of volume-reflective type holographic facets. As shown, thissystem design requires a somewhat different optical design in order toaccommodate the physics of such a volume-reflection scanning disc. Itwill be helpful to briefly describe the ray optics associated with theillustrative embodiment of such an alternative laser scanning systemdesign.

As shown in FIG. 44, each ray folding mirror 13A is provided with anaperture 120 and a first beam folding mirror 121. The function of thefirst beam folding mirror 121 is to direct the j-th aspect-ratiocontrolled laser beam from laser production module 12A, through theaperture 120, towards a second beam folding mirror 122 positioned in a“light ray free” region above the scanning disc. The function of thesecond beam folding mirror 122 is to direct the laser beam (1) towardsthe outer edge of the scanning disc to an incident point analogous to r₀in the design of scanning disc 7, described above. As the scanning discrotates, the j-th laser beam enters the volumetric depth of each i-thscanning facet, and as it reflects therefrom, it is diffracted in amanner determined by the fringe structure of the holographic designedtherein during the scanner design process. As the holographic facetrotates, the diffracted laser beam is reflected off its associated beamfolding mirror 13A so that the corresponding scanline P(i,j) is producedwithin the scanning volume of the scanner. Upon reflecting off a scannedcode symbol (or scanned textual character characters in holographic OCRapplications), the laser light scatters and a portion of the scatteredlaser beam reflects back along an incoming path that is spatiallycoincident with the outgoing path, as shown. As shown incoming lightrays A and B striking both the inner end outer edges of the scanningdisc at angles very close to the Bragg angle of the holographic scanningfacet, are strongly diffracted along optical paths (2) and (3) which aresubstantially parallel to the optical path (1) of the incident laserbeam. Consequently, a substantial portion of the optical power in theseincoming light rays is reflected from the scanning facet towards avolume-transmission hologram 123 which is supported above the scanningdisc adjacent the second beam folding mirror 122. The function of thevolume-transmission hologram 123 is to focus the collected light raystowards its focal point, at which the photodetector 15A is located.Notably, the size of hologram 123 is selected to collect all of thelight rays reflected off the holographic scanning facet, and itsposition is located within the light ray free region above the scanningdisc. All of the methods and procedures described above with regard tothe design and construction of volume-transmission scanning disc 7 aregenerally applicable to the design and construction of the holographicscanner of FIG. 44. In view of the above described teachings of thepresent invention, the HSD workstation of the present invention can bereadily modified for use in designing the holographic scanner of FIG. 44or any other holographic scanner of the present invention.

As can be imagined, the holographic laser scanner of the presentinvention can be used in diverse applications. While holographic laserscanner 1 has been described as a stand-alone, compact holographic laserbar code symbol reading system, it may in some applications be used as asubsystem within a larger scanning system, to simply detect the presenceof a code symbol within its robust scanning volume. As shown in FIGS.45A and 45B, holographic laser scanner 1 is used in just this way. Itsfunction is to simply detect the presence of a code symbol within itsrobust scanning volume and produce, as output, information specifyingthe position of the detected code symbol within the scanning volumeV_(scanning). Such information can be as simple as P(i,j) which, inessence, encodes (i.e. embodies) information regarding the focal planeand scanline within the focal plane along which a code symbol 130 movingalong conveyor belt 129 has been detected. In the example of FIG. 45A,the code symbol position information produced by holographic scanner 1is P(15,3), which specifies the scanline within the scanning volumewhich detected the code symbol. FIG. 5 shows which region within thescanning volume this particular scanline occupies. In the illustrativeembodiment of FIG. 45A, high-speed laser scanning system 131 has atranslation table stored within its on-board control computer which usesthe code symbol position information P(i,j) to produce information whichidentifies generally where the detected code symbol resides, i.e. interms of volumetrically-quantized regions of the scanning volumeV_(scanning). Laser scanning system 131 also comprises a high-speedlaser scanning mechanism which is capable of producing a laser beamhaving a variable depth of focus within the scanning volumeV_(scanning), and steering the laser beam to a specific regiontherewithin for aggressive scanning.

The exact sequence of steps undertaken during the operation of thescanning system shown in FIGS. 45A and 45B will be described below. Whencode symbol 130 is present in the scanning volume V_(scanning),holographic scanner 1 automatically detects this symbol and producesposition information P(15,3) which is provided to scanner 131. Aftertranslating this information to scanning region information, laserscanning system 131 uses the translated information to (i) set the focallength of the laser beam to the focal plane within which the detectedcode symbol has been detected (i.e. focal plane DF4), (ii) steer thelaser beam to the corresponding region within V_(scanning), and (iii)generate an X-bar or other scanning pattern within this region in orderto collect lines of high-resolution scan data within this region. Thecollected scan data is stored in a scan-data video buffer 131A and ahigh-speed decode processor 131B (i.e. microcomputer) decode processeseach frame of video data using stitching or other suitable symboldecoding techniques in order to read the scanned code symbol with thisregion of the scanning volume V_(scanning). Output symbol character dataproduced by processor 131B is then provided to the host computer system132. Then as the conveyor belt moves forward as shown in FIG. 45B, thenext package on the conveyor is brought through the scanning volume at ahigh speed. When code symbol 134 on this package is detected within thescanning volume, the above-described sequence of operations is carriedout once again. In this instance, however, the laser beam will beautomatically focused to the first depth of field (i.e. DF1), as this iswhere the detected code symbol resides as it passes through the scanningvolume. As such, the focused laser beam is automatically scanned withinthe small region defined by P(4,3) shown in FIG. 5. All other steps arethe same as described above. For each new package entering the scanningvolume, the code symbol(s) thereon is automatically detected andposition information related thereto provided to scanning system 131 tocause its scanning pattern to be directed to the region where thedetected code symbol momentarily resides for high-resolution scanning ofthis region.

As shown in FIG. 46, the holographic laser scanning system of thepresent invention can be easily scaled down in size and embodied withina fully-automatic, portable hand-supportable housing, a hand-mountedhousing, or body-wearable housing 140, having one-way RF signaltransmission capabilities, while retaining all of its essentialfeatures, namely: multiple focal planes within its scanning volume;non-astigmatic focal zones; and omni-directional scanning. In thisillustrative embodiment, the portable scanner of FIG. 46 embodies thefollowing functionalities: the spatially overlapping object detectionand laser scan fields taught in U.S. Pat. No. 5,468,951; thelong-range/short-range modes of programmable scanning operation taughtin U.S. Pat. No. 5,340,971; the power-conserving system-controlarchitecture taught in U.S. Pat. No. 5,424,525; and the RF signaltransmission functionalities and acoustical acknowledgement signallingtaught in copending U.S. patent application Ser. No. 08/292,237, each ofwhich is commonly owned by Metrologic instruments, Inc. of Blackwood,N.J., and is incorporated herein by reference in its entirety.

As shown in FIGS. 47 and 48, the holographic laser scanning system ofthe present invention can be easily modify, scaled down in size, andembodied within a fully-automatic, portable hand-supportable housing145, a hand-mounted housing 146, or body-wearable housing, havingone-way RF signal transmission capabilities. Notably, the primarydifference between the scanners shown in FIGS. 47 and 48 is that thescanner shown in FIG. 47 is hand-supportable, whereas the scanner shownin FIG. 48 is hand-mounted on the back-of-the hand using a fingerlessglove, as taught in copending application Ser. No. 08/489,305incorporated herein by reference.

In the illustrative embodiments shown in FIGS. 47 and 48, theholographic scanning apparatus of the present invention is used toproduce a 2-D raster-type of scanning pattern, with a depth of fieldextending from about 2″ to about 10″ from the scanning window of thescanner. As illustrated in FIG. 47, the scanner comprises avolume-transmission scanning disc 147 rotated by a smallbattery-operated motor 148 supported within the interior of the scannerhousing. The scanning disc has about twenty holographic facets, eachdesigned to produce one of the twenty scanlines (i.e. scanplanes) in the2-D raster scanning pattern within the 3-D scanning volume V_(scanning).As shown, a miniaturized laser beam production module 12A′, ashereinbefore described, is used to produce an incident laser beam freeof astigmatism and having a circularized or aspect-ratio controlled beamcross section. This laser beam is transmitted through a piezo-electriccontrolled Bragg cell 149 which directs the laser beam incident onto theunderside of the holographic scanning disc at any one of a very smallrange of incident angles Δθ_(i) determined by the scanning disc designprocess of the present invention described in great detail hereinabove.The function of the Bragg cell is thus to modulate the incidence angleof the laser beam about a center, or nominal angle of incidence θ_(i).The microprocessor based system controller (not shown) aboard thescanner generates control signals for the Bragg Cell during scanneroperation. When the laser beam is directed at the scanning disc at thenominal incidence angle θ_(i), it produces each one of the twentyprincipal scanning lines in the twenty-line raster scanning pattern asthe laser beam is diffracted by the twenty different holographicscanning facets. However, when the incidence angle is modulated aboutthe nominal incidence angle θ_(i), the diffracted laser beam is sweptabout an infinite, but small range of scanlines about its principalscanline causing “inter-scanline dithering”. If the deviation about thenominal incidence angle θ_(i) is symmetric, then the deviation in thediffracted scanlines will also be symmetric within the resulting rasterscanning pattern. Similarly, if the deviation about the nominalincidence angle θ_(i) is asymmetric, then the deviation in thediffracted scanlines will also be asymmetric within the resulting rasterscanning pattern.

In a manner similar to the facets in scanning discs 7 and 7′ describedabove, each scanning facet along scanning disc 147 also functions tocollect reflected laser light towards a small parabolic mirror 150having a focal point above the scanning disc near the motor, at whichphotodetector 151 is located. Intensity signals produced by thephotodetector 151 are provided to the microprocessor fordecode-processing in a conventional manner. An infra-red light basedobject detection transceiver 152 is mounted adjacent the scanning windowto produce the object detection field which spatially overlaps thescanning volume over its operative scanning range, as shown. In thisparticular illustrative embodiment, the portable scanner of FIGS. 4 and48 both embody the following functionalities: the spatially overlappingobject detection and laser scan fields taught in U.S. Pat. No.5,468,951; the long-range/short-range modes of programmable scanningoperation taught in U.S. Pat. No. 5,340,971; the power-conservingsystem-control architecture taught in U.S. Pat. No. 5,424,525; and theRF signal transmission functionalities and acoustical acknowledgementsignalling taught in copending U.S. patent application Ser. No.08/292,237, each of which is commonly owned by Metrologic instruments,Inc. of Blackwood, N.J., and is incorporated herein by reference in itsentirety.

Using the detailed design procedures described hereinabove, one withordinary skill in the art will be able to readily design a variety ofother different types of holographic laser scanning systems for use indiverse fields of utility.

While the various embodiments of the holographic laser scanner hereofhave been described in connection with linear (1-D) and 2-D code symbolscanning applications, it should be clear, however, that the scanningapparatus and methods of the present invention are equally suited forscanning alphanumeric characters (e.g. textual information) in opticalcharacter recognition (OCR) applications, as well as scanning graphicalimages in graphical scanning arts.

Several modifications to the illustrative embodiments have beendescribed above. It is understood, however, that various othermodifications to the illustrative embodiment of the present inventionwill readily occur to persons with ordinary skill in the art. All suchmodifications and variations are deemed to be within the scope andspirit of the present invention as defined by the accompanying claims toInvention.

What is claimed is:
 1. A bar code symbol scanning system for producing alaser scanning pattern having a plurality of scanlines over a pluralityof different focal planes within a 3-D scanning volume for scanning barcode symbols presented within said 3-D scanning volume, said bar codesymbol reading system comprising: a housing; a support disc disposedwithin said housing, being rotatable about an axis of rotation, andhaving an inner perimeter, an outer perimeter, and an available lightcollecting region defined between said inner perimeter and outerperimeter; a plurality of holographic optical elements for scanning alaser beam and producing a laser scanning pattern having a plurality ofscanlines over a plurality of different focal planes within a 3-Dscanning volume for scanning bar code symbols presented within said 3-Dscanning volume, each said holographic optical element being supportedon said support disc between the inner and outer perimeters, and eachsaid holographic optical element having a surface area for carrying outlight collecting operations, and at least a portion of said surface areabeing disposed adjacent said outer perimeter of said support disc forcarrying out laser beam scanning operations; a light focusing mirrordisposed beneath said support disc, for focusing light rays collected byeach said holographic optical element; and a photodetector disposedabove said support disc, for detecting the intensity of light rayscollected by each said holographic optical element, focused by saidlight focusing mirror, and transmitted through said holographic opticalelement onto said photodetector for detection and generation of anelectrical signal indicative of said detected intensity; wherein the sumof all of the surface areas of said plurality of holographic opticalelements is substantially equal to the surface area of said availablelight collecting region of said support disc; and wherein the amount oflight collected by each said holographic optical element and focusedonto said photodetector by said light focusing mirror is substantiallyequal during said light collecting operations.
 2. The bar code symbolscanning system of claim 1, wherein the refractive index of each saidholographic optical element has a variable spatial frequency over itssurface area, providing a focal length which is related to the distanceof the scanline to be produced by said holographic optical element. 3.The bar code symbol scanning system of claim 1, wherein the lightcollection efficiency of each said holographic optical element issubstantially equal.
 4. The bar code symbol scanning system of claim 1,wherein the inner surface boundary of at least one of said holographicoptical elements has an inner radius which is substantially greater thanthe inner perimeter of said support disc.
 5. The bar code symbolscanning system of claim 1, wherein each said holographic opticalelement is a volume transmission type hologram.
 6. The bar code symbolscanning system of claim 1, wherein each said holographic opticalelement is a volume reflection type hologram.
 7. The bar code symbolscanning system of claim 1, wherein the average refractive index of eachsaid holographic optical element is substantially equal over the entiresurface area thereof.
 8. The bar code symbol scanning system of claim 1,wherein the outer portion of the surface area of each said holographicoptical element used for scanning operations has a first averagerefractive index, whereas the remaining portion of the surface area ofeach said holographic optical element used for light collectingoperations has a second average refractive index.
 9. The bar code symbolscanning system of claim 8, wherein said first average refractive indexis different from said second average refractive index.
 10. The bar codesymbol scanning system of claim 1, wherein the light diffractionefficiency of said outer portion of said surface area is optimized for afirst polarization state of light, and the light diffraction efficiencyof said remaining portion of said surface area is optimized for a secondpolarization state of light orthogonal to said first polarization state.11. The bar code symbol scanning system of claim 10, wherein said firstpolarization state is the S polarization state, and said secondpolarization state is the P polarization state.
 12. A holographic laserscanning system for scanning code symbols, comprising: a housing; laserbeam producing means for producing a laser beam; and a laser scanningdisc disposed within said housing, having a total surface area availablefor light collection, and plurality of holographic optical elements forscanning said laser beam and producing a laser scanning pattern forscanning code symbols, wherein each said holographic optical element hasan optimized light collection efficiency and a light collection surfacearea which is maximized with respect to said total surface areaavailable for light collection.
 13. A holographic laser scannercomprising: laser beam producing means for producing a laser beam; asupport disc rotatable about an axis of rotation, and having an innerperimeter, an outer perimeter, and an available light collecting regiondefined between said inner perimeter and outer perimeter; and aplurality of holographic optical elements for scanning said laser beamand producing a laser scanning pattern for scanning code symbols, eachsaid holographic optical element being supported on said support discbetween the inner and outer perimeters of said support disc, and eachhaving a surface area for use in light collecting operations, whereinsubstantially all of the available light collecting surface area on saidsupport disc is utilized and the light collection efficiency of eachsaid holographic optical element is substantially equal.
 14. Aholographic laser scanner for scanning code symbols, comprising: a laserfor producing an outgoing laser beam; a support disc having an axis ofrotation, and an inner perimeter, an outer perimeter, and an availablelight collecting region defined between said inner perimeter and saidouter perimeter; and a plurality of holographic optical elements forscanning said laser beam and producing a laser scanning pattern forscanning code symbols, each said holographic optical element beingsupported on said support disc between the inner and outer perimeters ofsaid support disc, and each having a light collection area for use inlight collecting operations, wherein the size and shape of the lightcollection area of each holographic optical element is controlledindependent of the angular sweep of said outgoing laser beam in order tomake maximum use of the disk surface area for light collection functionsduring laser scanning operations.